Review ConditionMonitoringofRailTransportSystems: ABibliometricPerformanceAnalysisandSystematic LiteratureReview

MariuszKostrzewski1,*andRafaMelnik2

1 FacultyofTransport,WarsawUniversityofTechnology,00662Warsaw,Poland 2 FacultyofComputerScienceandFoodScience,LomzaStateUniversityofAppliedSciences, 18400oma,Poland;[email protected] * Correspondence:[email protected]

Abstract:Conditionmonitoringofrailtransportsystemshasbecomeaphenomenonofglobalinter estoverthepasthalfacentury.Theapproachestoconditionmonitoringofvariousrailtransport systems—especiallyinthecontextofrailvehiclesubsystemandsubsystemmonitoring—have been evolving, and have become equally significant and challenging. The evolution of the ap proachesappliedtorailsystems’conditionmonitoringhasfollowedmanualmaintenance,through methodsconnectedtotheapplicationofsensors,uptothecurrentlydiscussedmethodsandtech niquesfocusedonthemutualuseofautomation,dataprocessing,andexchange.Theaimofthis paperistoprovideanessentialoverviewoftheacademicresearchontheconditionmonitoringof railtransportsystems.Thispaperreviewsexistingliteratureinordertopresentanuptodate,con Citation:Kostrzewski,M.; tentbasedanalysisbasedonacoupledmethodologyconsistingofbibliometricperformanceanaly Melnik,R.ConditionMonitoring sisandsystematicliteraturereview.Thiscombinationofliteraturereviewapproachesallowsthe oftheRailTransportSystems: authorstofocusontheidentificationofthemostinfluentialcontributorstotheadvancesinresearch ABibliometricPerformance intheanalyzedareaofinterest,andthemostinfluentialandprominentresearchers,journals,and AnalysisandSystematic papers.Thesefindingshaveledtheauthorstospecifyresearchtrendsrelatedtotheanalyzedarea, LiteratureReview. andadditionallyidentifyfutureresearchagendasintheinvestigationfromengineeringperspec Sensors2021,21,4710. tives. https://doi.org/10.3390/s21144710

AcademicEditor: Keywords:conditionmonitoring;railtransport;railvehicle;track;monitoringsystem;sensor; MariaGabriellaXibilia bibliometricanalysis;mappingscience Received:12June2021 Accepted:6July2021 Published:9July2021 1.Introduction Publisher’s Note: MDPI stays Conditionmonitoringofcertaindevicesisappliedinaparticularsysteminorderto neutral with regard to jurisdic detectchangesoranalyzetrendsincontrollingparticularparameters,andmostofallto tional claims in published maps preventpotentialfailuresofasystem.Earlyproblemdiagnosisensurespreliminaryinter andinstitutionalaffiliations. vention for replacement ofany equipment’s components before serious consequences. The term “condition monitoring” evolved—in the case of research connected to rail transport—withthedevelopmentofinstrumentsthatallowthedevelopmentofincreas

Copyright:©2021bytheau ingly better measurement analyses. Such an evolution of terms, definitions, and ap thors.LicenseeMDPI,Basel, proachesconnectedtoconditionmonitoringinthecaseoftheaforementionedmodeof Switzerland.Thisarticleisan transportispresentedinTable1,basedon[1–45]. openaccessarticledistributed Recentdecadeshaveshownacertainevolutionofapproachestotheconditionmon underthetermsandconditions itoringofvariousrailtransportsystems,especiallyinthecontextofrailvehiclesandtrack oftheCreativeCommonsAttrib subsystems. The approaches applied to rail condition monitoring have ution(CCBY)license(http://cre evolvedfrommanualmaintenance,throughmethodsconnectedtotheapplicationofsen ativecommons.org/li sors,uptothecurrentlydiscussedtechniquesfocusedonthetermsofIndustry4.0.The censes/by/4.0/). enormous increase in the quantities of generated data—namely, big data—requires

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diagnosticprocedurestobeautomated.Suchachallengeinducedtheneedforuptodate and improved methods of autonomous interpretation of condition monitoring data (mainly vibration methods). This has led the authors to the aim of this review paper, whichistheconsiderationofseveralpiecesofresearchfocusedonthescientificareaof railtransportsystems’conditionmonitoring.Theauthorsofthisreviewpapertookinto considerationtwodistinctmethodsofscientificliteratureanalysis:ontheonehand,abib liometricperformanceanalysis(whosecomponentpartismetaanalysis)wascarriedout, andontheotherhand,asystematicliteraturereviewwasperformed.

Table1.Theoverviewofdefinitionsandapproachesconnectedtotheconditionmonitoringofrailtransportsystems. Source:ownelaborationbasedonreferencesdirectlymentionedinthetable.

Yearof Definition Source Publication “Theconditionmonitoringsystemsdevelopedsofarinrailvehicleapplicationsaremainly basedonthedirectmeasurementofrelevantsignals,whichareanalysedusingtimeand/or 2008 [45] frequencydomainsignalprocessing,e.g.,tofindfeaturesorsignaturesrelatedtoparticular faults[43,44]).” “(…)conditionmonitoringintoday’scomplexsystemsismostlyregardedasanalarmtoolfor maintenance[40].EventheISO17359defines,howaconditionmonitoringpolicyshouldlook 2009 [42] liketoestablishasuccessfulmaintenancestrategy[41].” “Conditionmonitoringsystemsareusedtocollectbothdigitalandanaloguesignalswithina locationroomutilizingdistributedtransducersconnectedtoeitherpointtopointordigital buscommunicationlinks.Theyoftencontainsomekindofalarmsystembasedaround thresholdingtechniques,thelimitsbeingsetbylocalmaintenancepersonnelwhohavetoin specttherecordedsignaturesdailytoattempttoanticipateanyfailures.Nodiagnosticcapa 2009 [39] bilitiesareprovided.(…)Faultdetectionanddiagnosissystemsareamoreadvancedversion ofconditionmonitoringsystems,incorporating‘intelligent’algorithmscapableofdetecting faultspriortofailure,diagnosingtheincipientfaultandprovidingsomeindicationofthecrit icalityofthedetectedfault.” “Conditionmonitoringisnecessaryinordertoimmediatelydetectvehiclefaults.Forcondi tionmonitoring,itisnecessarytodetectthefaultfromthesignalsofsensorsattachedtothe 2010 [38] vehicles[2].Conditionmonitoringcanbeconsideredtobeapartofthewellestablishedand welldevelopedareaofFaultDetectionandIsolation(orIdentification)(FDI).” “Conditionmonitoringmeasuresarecrucialtoensuresafeandcosteffectiveoperation intherailroadtransportationindustry.Awelldesignedmonitoringsystemsubstantiallyre 2012 [37] duceshardwaremaintenancecostandimprovesservicequalityandoverallsafety.” “Conditionmonitoringtechnologywithintherailwayindustryhasproliferatedinrecent years;thisisduetothecontinuousimprovementofelectronicbasedsystems.Thishascreated auniquesituationforimplementingproactiveconditionmonitoringtechnologyintherailway industry.Thisapproachwillcreatethepossibilitiesofidentifyingfailingsystemswhiletheas 2012 [6] setisinoperationbeforetheycreatecatastrophicdamage.Economically,mostoftheseproac tiveproductsarewaysideconditionmonitoringsystemsandveryfewsensorsarefewsensors aredirectlymountedonthevehicles.” “Conditionmonitoringofvehiclesallowstotrackthedevelopmentoftheirtechnicaldegrada tion,whichallowstoimplementrationalpreventiveandremedialactivitiesinordertoavoid unpredictabledowntimesrelatedtothedamagesandseriousfailures.Thereasonforcontinu 2013 [36] ousimprovementofvehicles’conditionmonitoringmethodsarestricterandstricterrequire mentsconcerningthereliabilityandsafetyofbothalltransportsystemandseparatemeansof transport.” “Conditionmonitoringrequiresthegatheringofdataandprocessingofthatdataintouseful 2014 [35] informationtosupportdesign,availability,reliabilityandmaintenancepractices.”

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“Conditionmonitoringaimstorecordthecurrent(realtime)conditionofasystem[33].” 2014 [34] “Conditionmonitoringdetectsandidentifiesdeteriorationinstructuresandinfrastructurebe forethedeteriorationcausesafailureorpreventsrailoperations.Insimpleconditionmonitor ing,sensorsmonitortheconditionofastructureormachinery.Ifthesensorreadingsreacha predeterminedlimitorfaultcondition,thenanalarmisactivated.However,thissimplistic approachmayleadtoalargenumberoffalsealarmsandmissedfailures[31].(…)Condition monitoringcanbeperformedcontinuouslyorperiodically.Continuousmonitoringshould 2014 [32] detectaproblemstraightawaybutitisoftenexpensive;energyhungry,whichisaproblem forWSNswherethenetworkcomponentsneedpower;andthesensordataareverynoisy, whichrequirescarefulpreprocessingtoensureaccuratediagnostics.Periodicmonitoringis cheaper,useslessenergy,andallowstimefordatacleaningandfilteringbutaproblemwill onlybediagnosedatthenextprocessingrun.(…)Inbasicconditionmonitoring,thesystemis onlyabletodistinguishbetweennormalandabnormalconditions(nofaultorfault).” “Underthisframework,theconditionbasedmonitoring(CBM),asatechniquetoprovide prognosisanddiagnosisofcomponentdegradation,allowstodetecttheinservicefailures andcontributestothedecisionmakingforimprovingthesystemperformance[19,26].Distin 2016 [30] guishedfromtheconventionalfaultdetectioncarriedoutinthedepot,thistechniqueisaim ingattheonlineidentificationofthecomponentperformanceontheoperationalconditions.” “Technologiesthatcanperformconditionmonitoringandconditionbasedmaintenanceare thusofsignificantinteresttoidentifyandpredictprogressingdegradationtrendsandopti 2016 [29] mallyschedulemaintenanceactionstolowermaintenancecostsanddowntime.” “Conditionmonitoringisreferredtoasaprocessofmonitoringtheconditionoftheobjective wheretherelevantparametersaremeasuredinordertodeterminethesignificantchanges whichisindicativeofadevelopingfault.Consideringtherailwaysystem,theobjectivestobe 2017 [28] monitoredcouldbe:[v]ehiclecomponentssuchaswheels,axlebearingsandbrakepads, [i]nfrastructuresuchasthetrack,railbedsandbridgesor[p]assengers/goodswithintrain cars.” “ConditionmonitoringcanbeconsideredtobeapartofthewellestablishedareaofFaultDe tectionandIsolation(orIdentification)(FDI)[25].Conditionmonitoringismainlyapplicable 2017 [27] tosystemthatdeteriorateintime.Theaimofconditionmonitoringistodetectandisolatede teriorationbeforeitcausesafailure[2,26].” “Railwaytrackandrollingstockconditionmonitoringisessentialinensuringthesafeandef ficientfunctionofrailwaysystems.Theabilitytouseaninstrumentedrevenuevehicleforthe conditionassessmentofrailtracksisparticularlysignificantbecausethiscapabilitywillnot 2017 [24] requiretrackaccessduringinspection.Thisinstrumentedvehiclecanalsoprovideusefuldata forthedefinitionoftherailtrackinputconditionsfortheassessmentofthestabilityofarail vehiclewhentraversingalongthetrack.” “Railwayinspectionisnormallyconductedperiodicallyeveryyearorseveralmonths.Itmay taketoomuchtimetorapidlydetectfaultsinthetrackthatmaycausecollapseorhugeloss, asisthecaseinthepromptidentificationofraildefects.(…)Hence,conditionmonitoringof 2018 [23] railinfrastructurehasbecomeimportantforsettingproperpredictivemaintenancesbeforede fectandfailuretakeplace.(…)Conditionmonitoringcanreducemaintenanceanditscostsby detectingthefaultsbeforetheycancausedamageorpreventrailoperations[22].” “Conditionmonitoringisdefinedasthepracticeofmonitoringparametersofasystemor component,withtheintentofgettingdeeperknowledgeofhowthatsystemchangesandde 2018 [21] teriorateswithtime.Ifkeyparametersthatcanbelinkedtowear,faultsanddegradationcan bemonitored,emergingproblemscanquicklybefoundorevenpredicted.” “Conditionmonitoringisbasedontheregularacquisitionofmachineconditionsortheircom ponentsbymeasuringandanalyzingphysicalquantities.Thesearecomparedbythesystem 2018 [20] withspecifiedconditionsandservefordiagnosisdecisions[14,18,19].”

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“Conditionmonitoringisasuitablewaytodetectimpendingvehiclefailuresandacoustically criticaltramsimmediatelybutundertheaspectofconservationofresources(exploitationof wearreservesoftribologicalcomponents).Inordertoimplementsuchconcepts,thetramop 2019 [17] eratorhastobeabletoquicklyandaccuratelyrecordthecurrenttechnicalconditionoftheve hicleinordertoinitiateappropriatecounteractionsandkeepdowntimestoaminimum.” “Abetterwaytoavoidbreakdownsisamaintenancestrategythatmonitorsandreportin realtimetheconditionofamachineordeviceinuse,akeyprincipleofIndustry4.0,soitsre maininglifecanbeestimated.Thisiscalled‘conditionmonitoring’and‘diagnosticengineer 2019 [16] ingmanagement’.Researchershavenotedconsiderableevidencethat‘conditionbased maintenance’giveseconomicadvantagesinmostindustriesandisthebestavailablestrategy forpreventingunexpectedsystemdowntime[11–15].” “Theconditionmonitoringofrailwayvehicleshastraditionallyreliedonsignalprocessing andknowledgebasedtechniquesbut,ontheotherhand,modellingtechniquesgivegreatpo 2019 [10] tentialsduetotheaprioriknowledgeincludedinthemodel.” “Modernconditionmonitoringsystemshaveledfaultdiagnosistoentertheeraofbigdata, 2019 [9] whichleadtotheobsolescenceoftraditionalphysicsbasedfaultdiagnosismethods[8].” “Conditionmonitoringofrailwayvehicleandtrackdynamicsaremonitoredviatrackbased systemsorvehiclebasedsystems([6][–actualizedbytheauthors].Systemsthataretrack basedareusuallyusedformonitoringtheconditionofwheelsets,wherevehiclebasedsys temsareusedtomonitortheconditionofthebogie,suspensionandridecomfort.(…)Condi 2020,2012 [6,7] tionmonitoringsystemsaremadeupofsensorsandcollectivedataprocessinghardwareare softwarethisallhastobemountedontothedesiredrollingstockinthecorrectlocationto gainthedesiredresults.” “(…)theconditionofcomplexsystemsistypicallymonitoredbyalargenumberofdifferent typesofsensors,capturinge.g.,temperature,pressure,flow,vibration,imagesorevenvideo 2020 [5] streamsofsystemconditions,resultinginveryheterogeneousconditionmonitoringdataat differenttimescales.” “ConditionMonitoring(CM)isanefficientandachievablewaytoensurethereliabilityofsus pensionsystems,therefore,numerousscholarshavefocusedondevelopingsuitableap 2020 [4] proachestomonitorthesuspensionsystems([2,3]).” “(…)twogenerallyconcernedquestionsofconditionmonitoring[arethesubjectofresearch ers’interest—addedbythepaper’sauthors]:(i)evaluatingthediagnosabilityandisolatability 2020 [1] ofasystemwithexistingsensornetwork;and(ii)findingoutaquantityoptimumsetofsen sorsforadesireddiagnosabilityandisolatabilityofasystem.”

1.1.Background Accordingtotheinformationcontainedinkeyscientificdatabases,nearly20review papersontheconditionmonitoringofrailroadsystemshavebeenpublishedinthelast4 decades.Thisisaninsignificantnumberconsideringtheessenceofthesubjectmatter; nevertheless,itisworthunderliningthatthisnumberconcernssomelimitedareasthat willbedefinedlaterinthecurrentarticle.Byrecallingthepublishedreviewpapers,itcan bestatedthattheirauthorshavetakenunderconsiderationtopicssuchas(certainly,the belowmentionedtopicswereconsideredinresearchpapersaswell;however,sincere viewpapersaretreatedascertainsummaries,researchpapersareomittedinthefollowing list): conditionmonitoringtechniquesforrailvehicledynamics[10], applicationsandvarioussoftcomputingmethodsusedforconditionmonitoringin thecontextofintelligentsystems[46], descriptionofonboardconditionmonitoringsystems,methods,anddevices—with aparticularinterestinmicroelectromechanicalsensors,microprocessors,andtrans ceivers—creatingwirelesssensornetworksforfreightrailtransport[47],

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methods,techniques,andapplicationsconnectedtotheconditionmonitoringofrail roadswitchesandcrossingsystems[48], conditionmonitoringsystemsandenablingautomaticrailroadtrackinspection[49], railroadtrackconditionmonitoringwiththeuseofinertialsensors(inertialmeasure mentsensors)andGPSsignals[50], briefdiscussiononmachinelearningtechniquesinrelationtorailtrackcondition monitoring[51], railwayturnoutconditionmonitoring[52,53]), variousaspectsconnectedtotheindustrialInternetofThings solutionsrelatedto conditionmonitoring[54], varioustypesofsensorsinwiredandwirelesssensornetworks[23],andwireless sensornetworkbasedconditionmonitoringappliedinthecaseoftherailindustry [32,55], conditionmonitoringassociatedwithfaultdiagnosisofrollingbearings[56]. Thisbrieflypresentedbackgroundallowsustostatethatnocomplexreviewpapers connectedtoconditionmonitoringforrailtransportwerenoted.Asthiscanbetreatedas aresearchgap,theauthorsdecidedtoconsideranindepthstudyconsistingofacomplex reviewofapproachestoconditionmonitoringforrailroadtransport,underparticularas sumptions.Thisstudyisrelatedbothtopassengerandfreightrailroadtransportsystems’ conditionmonitoring—inaspectsconnectedtobothrailvehiclesandtracks.

1.2.TheAimoftheResearchandResearchQuestions Aswasobserved,thereisalackofreviewpapersthattreatconditionmonitoringof railwaytransportsystemsascomplexsystemscomposedofvariousfacilitiesandsubsys tems,straightforwardlydiscussingbothrailtracksandrailvehicles.Thisreviewpaper’s authorshavetriedtofillsucharesearchgap.Additionally,wedecidedtotakeintocon siderationthepastfewdecadesofconditionmonitoringconsiderationspresentedinnu merouspublications,whichledustoformulatethemainresearchaimsofthispublication. Theresearchquestionsareasfollows: RQ1:Whatarethecurrenttrendsinconditionmonitoringapproaches,andhowhave thesetrendsevolvedoverthelastdecades? RQ2:Whatarethefutureresearchdirectionsandperspectivesintheconditionmon itoringofrailtransportsystems? Thispaperis organizedaccordingtothefollowingstructure:Thebackgroundre sultedinresearchquestionsthatneedtobeverifiedbasedonacertainmethodology.The authorspresentthemethodologyofthecurrentresearchinthenextsection.Thetwosec tionsfollowingthedescriptionofthemethodologywereselectedbasedonmethodologi calapproaches,andareasfollows:bibliometricperformanceanalysis,andsystematiclit eraturereview.Finally,inthelastsection,wediscussthemainresultsofthisstudy,and additionallypresentcurrentresearchlimitationsandfutureresearchagendas.

2.Methodology Thetypeofresearchmethodologyandmannerofdatacollectionforfurtheranalysis dependontheresearchquestionbeingstudied.Therefore,beforewepresentourresearch methodologyappliedinthecaseofthecurrentpaper,weshallpresentpotentialwell knownmethodsandtechniquescommonlyusedintheliteraturereviewcontext.Conse quently,apropercompilationofmethodsandtechniquesispresentedattheendofthis section. Thispaperisbasedoncoupledreviewmethodologies—itcombinesabibliometric performanceanalysisbasedoncontentbasedanalysis,andasystematicliteraturereview asaresultofthecontentbasedanalysis.Thefirstofthesemethodsisaformofquantita tiveanalysis,whilethesecondisqualitative.Asawhole,thismethodologyadequately

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presentsthefullspectrumofpublicationsconnectedtotheconditionmonitoringofrail transportsystems,bothinquantitativeandqualitativeterms. Neuendorf[57]definedcontentanalysisasasystematic,objective,quantitativeex aminationofaparticularmessage’scharacteristics.Variousmethodsandtechniquesof contentanalysisaregiveninMayring[58].Contentbasedanalysisisaspecificresearch techniquefordatafeedingappliedinordertodeterminetheoccurrenceofcertainwords, phrases,keywords,themes,orconceptswithinsomeselected,thematicallycoupledqual itativedata.AccordingtoHsiehandShannon[59],threevariousapproachestocontent analysis were verified: conventional, directed, and summative. Working with conven tionalcontentanalysisstartswithobservation,andparticularcodesareobtainedbasedon dataanalysis.Meanwhile,directedcontentanalysisstartsresearchonaspecifictheory, andresearchfindingsappropriatetothetheorythathelptoobtaininitialcodesintended forlaterresearch,whereassummativecontentanalysisisconnectedtointerpretationof datasuchaskeywords,andfurthercountingandcomparisonsofkeywordsorothercon tent.Inthecaseofthelatterapproach,keywordsderivefromresearchers’interestorfrom literaturereview.Basedonsuchbriefdefinitions,thelatterapproachwasidentifiedasthe supportingoneforthepurposesofthispaper.Theobtainedkeywordsandelaborated resultsofcontentanalysisservedasafeedstockforfurtherdetailedresearch(i.e.,system aticliteraturereview).Inthecaseofresearch,suchsourcesofdata/feedstockcanbebooks, essays,discussions,newspapers,speeches,media,historicaldocuments,etc.Inthisre viewpaper,thesedocumentsarescientificpapers,conferencepapers,books,bookchap ters,notes,editorials,letters,shortsurveys,andconferencereviews.Thisisanopencata logofpublicationtypes;however,itscomponentsarecarefullyselectedinordertoanswer researchquestionsaspreciselyaspossible. Abibliometricperformanceanalysisbasedoncontentbasedanalysisisoneaspectof thecurrentpaper,whiletheotheristhesystematicliteraturereview.Literaturereviewin generalischaracterizedbycertaintypology,whichwaspresentedindetailinMarczewska andKostrzewski[60]basedonGrantandBooth,PetticrewandRoberts[61,62].Aswas mentionedinPetticrewandRoberts[62],asystematicreview’saimis“toprovideanob jective,comprehensivesummaryofthebestevidence”.Theauthorsofthispaperconsid eredthissystematicliteraturereviewasacombinationofanarrativereview(treatedas thesynthesisofselectedresearch,giveninmostcasesasadescriptiveratherthanstatisti calspecificationPetticrewandRoberts[62])andacriticalreview(comparisonandevalu ationofvariousperspectivesofaparticularresearchproblem). Thedatasearchingprocesswaslimitedtocontributionspublishedbetween1980and 2020(theinitiatingsearchwasperformedon29November2020;however,itmustbemen tionedthatthefinalsearchwasupdated—informationaboutwhichisdiscussedinfurther partsofthispaper).Theenddatewasbasedonthecontentofthescientificdatabases.The specificinformationconnectedtoeverystageoftheresearchreviewisprovidedinthe followingsubsections.TheprocedureofthisresearchreviewisshowninFigure1.

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Figure1.Theprocedureoftheconductedresearchreview.Source:basedon[58](p.89),[60].

3.BibliometricPerformanceAnalysis Thefirststageofthesearchprocesswasnotlimitedtoanyperiodoftime(however, datacollectionanalysesweresummarizedon1July2020,andthentheactualizationwas obtainedon29November2020,and8January2021). Sinceourinterestwasfocusedontheconditionmonitoringofrailwayvehiclesand tracks, thefollowingset ofkeywordswasinvestigated:“condition”*AND“monitoring” AND“railway*”AND“track”OR“vehicle”.Thisresultedinthereturnof6115recordsin total in the Scopus database (5830 publications before and 285 after actualization). It shouldbementionedthat23publicationsintotalwereassignedtotheyear2021,andthese areincludedinthisnumber,butexcludedfromthebelowstatistics.Themajorityofthe publicationswerereleasedinEnglish,andasignificantnumberoftheminChinese—all

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ofthelanguagesofthepublicationsaregiveninFigure2.Whenthelanguageofthepub lications was taken into consideration, we decidedtoanalyze onlythe publications in English(especiallygiventhatalmost92%ofallofthepublicationswerepreparedinEng lish).ItwasdecidedtoexcludelanguagesotherthanEnglish,sincethislanguageistreated asaninternationalonefortheexchangeofresearchandopinionsbetweenscientists,and forscholarlycommunication[63].Thedatabasecode“(condition*ANDmonitoring)AND (railway*)AND(track*)AND(LIMITTO(LANGUAGE,“English”))”allowedustolimitthe numberofsearchedpublicationsto5185items(4946publicationsbeforeand239afterac tualization).Onceagain,itshouldbementionedthat23publicationswereassignedtothe year2021,andtheseareincludedinthisnumber,butnotincludedinthebelowstatistics.

Figure2.Percentageofpublicationsperlanguage.

AllofthepapersinEnglishwerealsofilteredbasedontheirtype.AsshowninFigure 3,themajorityofthepublicationswerescientificarticles(55.1%)andconferencepapers (31.6%).

Figure3.Percentageofpublicationsperdocumenttype.

Thenumberofpublicationsintheresearchareaofconditionmonitoring,releasedin English,grewannually.Thenumberofpublications(i)inthementionedresearchareaper year(y)ispresentedinFigure4.AscanbeassumedbasedonFigure4,thefirstwaveof

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interestinrailtransportconditionmonitoringwasattheendofthe20thcentury(fromthe early1980s).Assumingthat100publicationswouldbetheparticularthreshold,itcanbe observedthatthesecondwaveofinterestwasbetween2000and2007.Inthecurrentpe riodoftimei.e.,2008–2020,theinterestisenormouswhennumbersaretakenintoconsid eration.Whatmightalsobeobservedisthatsince1994therehavebeennogapyearsin thepublishingofmanuscriptsinEnglishconnectedtotheanalyzedresearcharea.The increaseinthenumberofdocumentspublishedperyearbetween1982and2020canbe determinedbyanexponentialequationwithacoefficientofdeterminationequaltoR2= 0.964;thisequationisnotedasEquation(1).Theendoftheyear2020wasnotexcluded fromthisanalysis;however,oneshouldbearinmindthatthenumberofpublicationswill increase,consideringthatindexinginscientificdatabasesisoutofregulations;forexam ple,iftherewasstillamonthof2020leftuntiltheendoftheperiodbetweenthecomple tionoftheanalyzeddataandtheendoftheyear,notallofthereleasedpublicationswould havebeenindexedinthedatabaseyet.

8 10 . (1)

Figure4.Numberofpublications(i)intheresearchareaofconditionmonitoringperyear(y).

IntheScopusdatabase,eachpaperisclassifiedbysubjectarea.Inthecaseofthe analyzedresearcharea,onewouldconsidersomeofthesubjectareasmentionedinFigure 5tobeatypical.Nevertheless,wedecidednottolimitpublicationsaccordingtoanysub jectarea;however,Engineeringwouldfirstandforemostbetheareamostthematically connectedtothesubjectmatter,andsecondlythemostnumerousofallintermsofnumber ofpublications.Instead,wedecidedtolimitthepublicationsbasedonScopusdatabase internalkeywords—internalkeywordsareunderstoodasthosethatwerenotdecidedby thecurrentlyexaminedpublications’authors,butratherbytheemployeesofthecompany thatownstheScopusdatabase.ThesekeywordsarementionedinFigure6a–c(therewere 160Scopusdatabaseinternalkeywordsverified;therefore,afigureshowingonegraph onlywouldbeimpossibletoreadindetail—wethusdecidedtopresentitinthreeseparate graphsindicatinganumberofpublicationsassignedtointernalkeywordsequaltobe tween0and64(partc),between65and99(partb),andover100(parta).

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Figure5.Numberofpublications(i)intheresearchareaofrailwaytransportconditionmonitoring perparticularsubjectarea.

(a)

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(b)

(c) Figure6.Numberofpublications(i)intheresearchareaofrailwaytransportconditionmonitoring perparticularScopusinternalkeywords:anumberofpublicationsassignedtointernalkeywords equaltobetween0and64(c),between65and99(b),andover100(a).

PotentialScopusdatabaseinternalkeywordsofinterestforthepurposesofthispaper wouldbe“railroad”,“railroadtransportation”,“rails”,“conditionmonitoring”,“moni toring”(includedin“conditionmonitoring”term),“railway”,“faultdetection”,“railway track”,“vibrationanalysis”,“damagedetection”,“failureanalysis”,“railwayinfrastruc ture”,“faultdiagnosis”,“vibration”,“monitoringsystem”,“railwayaccidents”,“surface defects”,“trackirregularity”,“accidentprevention”,“railroadengineering”,“condition monitoringsystem”(thistermisalsounderstoodas“conditionmonitoring”),“trackge ometry”,“railtrack”,“rail”,“railways”,etc.Althoughinternalkeywordssuchas“rail road”,“railroadtransportation”,and“rails”describemorepublicationsthan“condition monitoring”terms,thesearetoogeneral,andarecharacterizedbyahighdegreeofab stractnessinconnectiontotheparticularsubjectmatter.Ontheotherhand,theremaining internalkeywordsareassignedtofewerpublicationsthan“conditionmonitoring”.There fore,weassumedthattheresearchpublicationsofinterestwerehiddeninagroupfocused ontheinternalkeyword“conditionmonitoring”.Therefore,publicationsfromthecircle ofthiskeywordaresubjectedtoanalysisinthisarticle. Atotalof617publicationsassignedtotheconditionmonitoringtermwerereleased in English between 1982 and thefirst half of 2020(as mentionedabove, somegeneral

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researchofliteraturewasundertakenon1July2020,andthentheactualizationwasob tainedon29November2020—sincewetookintoconsiderationthedirectinternalkey word“conditionmonitoring”,theliteraturereviewwasfinallyactualizedon8January 2021,andhenceforth,inthispublication,thewholeperiodfrom1982totheendof2020is assessed).Mostofthecontributionswereclassifiedasarticlesandconferencepapers,re views,andconferencereviews;hence,thesetypesofreferenceswereofinteresttous.Un likethesamplebeforefilteringviainternalkeywords,morepublicationsinthissample werereleasedasconferencepapers(54.9%)thanarticles(41.0%)(Figure7). Tosatisfythereader’sinquisitiveness,thefullcodeappliedintheScopusdatabase searchengineisasfollows: (condition* AND monitoring) AND (railway*) AND (track*) AND (EXCLUDE (PUBYEAR,2021))AND(LIMITTO(LANGUAGE,“English”))AND(LIMITTO(EXACT KEYWORD,“ConditionMonitoring”)). Notesandconferencereviewswereexcludedfromtheresearch,sincethesearebe lievedtobelessinfluentialinscientifictermsand,thus,lessrelevant.Forthisreason,the studiestobeassessedwereselectedfromtheseareas,constitutingafinalresearchsample of613documentresultsfoundwiththefollowingfullcodeappliedintheScopusdatabase searchengine: (condition* AND monitoring) AND (railway*) AND (track*) AND (EXCLUDE (PUBYEAR,2021))AND(LIMITTO(LANGUAGE,“English”))AND(LIMITTO(EXACT KEYWORD,“ConditionMonitoring”))AND(EXCLUDE(DOCTYPE,“cr”)OREXCLUDE (DOCTYPE,“no”).

Figure7.Percentageofpublicationsperdocumenttypefortheinternalkeywordsconnectedtocon ditionmonitoring.

Itisworthexplainingthechoiceofthescientificdatabaseintheseconsiderations.A coupleofyearsago,theauthorsofBaierFuentesetal.[64]mentionedthattheWebof Science(WoS)databasewastheprimarybibliometricsourceofscientificevaluation.In recentyears,theScopusdatabasebecamepredominanttoWoS.TheScopusdatabaseper formssimilarbibliographicattributesasWoS,suchasliteraturesearchingandcitation analysesofbibliometricrecords.Forconfirmationofthevalidityandadequacyofthein vestigations,bothdatabasesweresearchedbeforefurtheranalysis.Similarlytosearches oftheScopusdatabase,wealsofoundrecordsontheWoSdatabase.Sinceourinterest wasfocusedontheconditionmonitoringofrailwaytracks,weinvestigatedthesameset keywordsonWoS,i.e.,:“condition*ANDmonitoring”AND“railway*”AND“track”OR “vehicle”. The full code applied in the WoS database search engine was as follows:

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((TS=(condition*ANDmonitoring)ANDTS=(railway*)ANDTS=(track*))OR(TI=(condition* ANDmonitoring)ANDTI=(railway*)ANDTI=(track*))OR(AB=(condition*ANDmonitoring) ANDAB=(railway*)ANDAB=(track*))OR(AK=(condition*ANDmonitoring)ANDAK=(rail way*)ANDAK=(track*)))ANDLANGUAGE:(English)ANDLANGUAGE:(English)Refined by:DOCUMENTTYPES:(ARTICLEORREVIEWORBOOKCHAPTERORPROCEED INGSPAPER)Timespan:Allyears.Indexes:SCIEXPANDED,SSCI,A&HCI,CPCIS,CPCI SSH,BKCIS,BKCISSH,ESCI,CCREXPANDED,IC(itshouldbenotedthatthesearch acronyms’meaningsareasfollows:AK—authors’keywords;TS—topic;TI—title;AB— abstract).ThesearchoftheWoSdatabaseresultedin473records(checkedon8January 2021).ThemajorityoftheWoSindexedpublicationswerealsofoundontheScopusdata base.Moreover,theScopusdatabaseischaracterizedbyahighernumberofuniquejour nalscomparedtoWoS,asaresultofwhichthetotalnumberofrecordsintheScopus databasewasgreaterthanintheWoSdatabase.Acomparisonoftherecordsfoundin bothdatabasesisgiveninFigure8(pleasenotethatthetypeofpublicationsknownin ScopusasconferencespapersareknowninWoSasproceedingpapers).Takingallofthe aboveintoaccount,theScopusdatabasewasselectedasthemainsourceofbibliometric recordsforthestudypresentedinthisreviewpaper.

Figure8.NumbersofpublicationsperdocumenttypefortheScopusandWoSdatabases.

Theincreaseinthenumberofpublicationsfromoneyeartoanotherisdifferentwhen theconceptofpublicationsislimitedonlytothoseforwhichtheinternalkeyword“con ditionmonitoring”wasindicated.ThereisnoexponentialdependenceasseeninFigure 3;instead,theincreaseinthenumberofpublicationsischaracterizedbyapolynomial trend.Atrendlinewascomputedsince1995(Figure9);thisshowsthatresearchcontinued onaregularbasis,butwasnot“burdened”withtheinfluenceoftheinterestamongre searchersincertainperiodspecifictermsorkeywords.Theincreaseinthenumberofdoc umentspublishedperyearbetween1995and2020canbedeterminedbyapolynomial equationwithacoefficientofdeterminationequaltoR2=0.9147;thisequationisnotedas Equation(2).

0.1302 519.86 518956 (2)

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Figure9.Numberofpublications(i)intheresearchareaoftrackconditionmonitoringperyear(y) fortheinternalkeywords“conditionmonitoring”.

Currently,theparticularpublicationcouldbeassignedtoseveralsubjectareas,which meansthatpublicationsassignedtotheEngineeringsubjectareacanbeassignedtosev eralotherareasaswell.Thisisbasedlessonthedecisionofaparticularpublication’s author(s),butmoreonanalgorithmofthedatabase.Ascanbeobserved,inthecaseofthe “conditionmonitoring”internalkeywords,mostofthepublicationsareindexedinthe Engineeringsubjectarea,asisshowninFigure10.Itshouldbeunderlined,onceagain, thatthisanalysisislimitedonlytopublicationsforwhichoneoftheinternalkeywords wasindicatedas“conditionmonitoring”.

Figure10.Numberofpublications(i)intheresearchareaofrailwaytransportconditionmonitoring perparticularScopusresearchareafortheinternalkeywords“conditionmonitoring”.

Cooperationinpublicationandcitationoftheresearcherscitedinthecaseofthein ternalkeywords“conditionmonitoring”isoneofthemostimportantanalysesbywhich topresentthelevelofresearcherproductivity.Inordertoconductsuchananalysis,itwas

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importanttounifydataimportedfromthedatabase.Thesetofdataconnectedtothepub lishedmanuscriptswerenotgivenwiththeuniquenamesorsurnamesoftheselected researchers.Therefore,beforeanalysis,thefollowingunificationswereperformed:every time“GoodallR.M.”appearedinthedatafileitwaschangedto“GoodallR.”;“Weston P.F.”to“WestonP.”;and“GarcíaMárquezF.P.”to“MárquezF.P.G.”,etc.Afterthiskind ofcorrection,itwaspossibletoobtainanetworkofcooperationinpublicationandcitation ofresearcherscitedinthecaseoftheinternalkeywords“conditionmonitoring”;Thisnet workispresentedinFigure11.Toobtainthisgraphitwasassumedthattheminimum numberofdocumentsindexedinthedatabaseperauthorintheanalyzedperiodhadto beequaltofive.Theminimumnumberofcitationswasnotlimited.1268authorswere analyzed,andonly57metthethresholdfornumberofpublisheddocuments(ifweset thisthresholdtoaminimumof4documents,thenumberofauthorsmeetingexpectations wouldbe84;for3documents,itwouldbe144authors;andfor2documentsitwouldbe 304authors). Forthetotalnumberof57uniqueresearchers,thetotalstrengthofcoauthorship linkswithotherauthorswascalculated(Table2;includingthelistoftheresearchers’pub lications).Theresearcherswiththehighestlinkstrengthwereselectedforgraphicalinter pretation,totaling25authors.

Figure11.Cooperationinpublicationandcitationoftheresearcherscitedinthecaseoftheinternalkeywords“condition monitoring”.Source:OwnstudyappliedwiththeuseoftheVOSviewersoftware(version1.6.15).

ThelargerthediameterofacertainnodeaspresentedinFigure11,thehigherthe strengthofcooperationwithotherresearchers.Ascanbeobserved,alloftheresearchers weredividedintosixgroups(sixclusters).Itcanbenoticedthatthegreatestproductivity wasthatofRobertsC.(thediameterofthenodeassignedtothisresearchershowsthe greatestproductivityintermsofpublications),whocanbeadditionallydistinguishedby havingthehighestnumberofcoauthorshippublications(thisresearcherwascoauthor in34publicationsand748citationsbythetimeofdatacollection).Leadingpositionsin otherclustersareinthecasesofPapaeliasM.(12publicationswith605citationsintotal), GoodallR.(15publicationsassociatedwiththehighestnumberofcitationsintotal,i.e., 814),BruniS.(15publicationswith284citationsintotal),TsunashimaH.(13publications with274citationsintotal),andMatsumotoA.(9publicationswith48citationsintotal).

Table2.Cooperationinpublicationandcitationoftheresearcherscitedinthecaseoftheinternalkeywords“condition monitoring”,obtainedusingVOSviewersoftware(version1.6.15).

TotalLinkStrength Numberof Numberof Computedinthe ListofDocumentsPublishedbyaResearcherina Author Documents Citations VOSviewer ParticularYear Published Obtained Software 2018:[65–67],2017:[68,69]),2016:[70],2014:[71], AkinE. 8 119 14 2013:[72]

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2019:[73,74],2017:[75],2016:[30,76,77],2014:[78], AlfiS. 11 67 11 2012:[79,80],2011:[81],2008:[82] AnyakwoA. 5 22 13 2013:[83],2012:[84,85],2011:[86–88] AsanoA. 4 7 11 2016:[89,90],2014:[91,92] AsplundM. 5 35 8 2016:[93–95],2014:[34,96] AydinI. 5 97 10 2018:[66],2017:[68],2016:[97],2014:[71],2013:[72] 2017:[98],2015:[99],2013:[83],2012:[6,84,85,100], BallA. 8 119 15 2011:[86–88] 2020:[101],2019:[73,74],2017:[75],2016: BruniS. 15 284 14 [30,76,77,102],2014:[78],2012:[79,80],2011:[81],2008: [82],2007:[26],2006:[103] CapolinoG.A. 6 138 5 2017:[104],2011:[105],2009:[106,107],2008:[108,109] ColeC. 5 20 0 2019:[47],2018:[110–112],2009:[113] 2019:[48,114],2013:[115],2012:[116,117],2011:[19], DixonR. 9 156 17 2010:[118,119],2008:[33] 2019:[120],2018:[121,122],2017:[123,124],2016:[125], DollevoetR. 11 95 15 2015:[126–128],2014:[129],2008:[130] FirlikB. 6 19 0 2020:[131],2012:[132–136] 2016:[137],2015:[138,139],2014:[34],2013:[140], GalarD. 6 91 5 2012:[141] GaoM. 5 40 11 2020:[142],2019:[143],2018:[144,145],2017:[146] Lalletal.(2012a)[147],Lalletal.(2012b)[148],Lallet GoebelK. 5 134 10 al.(2011a)[149],Lalletal.(2011b)[150],Lalletal. (2010)[151] 2013:[115],2012:[116],2011:[19,117],2010:[118,119], GoodallR. 15 814 36 2009:[152],2008:[33],2007[25,153–155],2006:[2,156], 2004:[157] GoodmanC.J. 5 202 16 2009:[158],2007:[153–155],2006:[159] GuF. 6 74 11 2017:[98],2012:[6,85,100],2011:[86,88] HenaoH. 5 122 5 2011:[105],2009:[106,107],2008:[108,109] HoS.L. 5 64 0 2012:[37],2008:[160,161],2006:[162,163] HuangZ. 5 22 6 2017:[164,165],2016:[166,167],2014:[168] KaewunruenS. 7 44 3 2019:[169,170],2018)[23,171],2017:[164,172,173] 2020:[174],2018:[65–67,175],2017:[68,69],2016: KarakoseM. 10 94 17 [70,97],2014:[71] KostrzewskiM. 5 52 0 2018:[176,177],2017:[178],2014:[179],2012:[180] KumarU. 6 14 5 2016:[137,181],2014:[182],2012:[141,183],2010:[184] LallP. 5 134 10 2012:[147,148],2011:[149,150],2010:[151] LiP. 5 411 16 2007:[153–155],2006:[159],Lietal.(2004)[157] 2020:[185,186],2019:[187,188],2018:[144],2017:[104], LiY. 8 37 5 2016:[12],2011:[189] 2020:[190],2019:[187,191],2018:[56,121],2017: LiZ. 14 152 14 [123,124],2016:[125],2015:[126–128],2014:[129,192], 2008:[130] 2020:[193],2019:[194,195],2018:[196],2016:[197], LiuX. 6 9 5 2014:[198] LoweR. 5 134 10 2012:[147,148],2011:[149,150],2010:[151] MarkineV.L. 6 9 5 2020:[193],2019:[195],2018:[196,197],2014:[198,199] 2020:[200],2018:[201,202],2014:[92],2012:[203], MatsumotoA. 9 48 23 2011:[204],2008:[205,206],2006:[207]

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2014:[208],2011:[19],2009:[45],2008:[209,210],2007: MeiT.X. 6 347 9 [26] MoriH. 6 38 15 2016:[89,90],2014:[91,92],2011:[38,204] 2017:[211],2016:[212,213],2015:[214,215],2012: MárquezF.P.G. 13 759 10 [216,217],2011:[218],2010:[39,219],2008:[220],2007: [221,222],2003:[223] NaganumaY. 5 50 6 2014:[224,225],2013:[226],2011:[204],2008:[227] 2019:[120],2018:[122],2017:[123,124],2016:[125], NúñezA. 9 63 12 2015:[126–128],2014:[129] OhnoH. 5 30 15 2020:[200],2018:[201,202],2012:[203],2006:[207] 2016:[93–95],2014:[34,96],2013:[228],2012:[141,183], PaloM. 9 52 9 2012:[229] 2020:[230],2019:[170],2017:[164,165,211,231],2016: PapaeliasM. 12 605 14 [166,213],2014:[167,168],2012:[216],2011:[218] PislaruC. 7 87 15 2014:[232],2013:[83],2012:[6,84,85],2011:[86,88] RantataloM. 6 25 7 2019:[233],2016:[93–95],2014:[96,182] 2020:[230,234],2019:[235,236],2017:[237],2015: [214,238,239],2014:[240–243],2011:[19,244–246], RobertsC. 34 748 53 2010:[39,219,247],2009:[152,158,248],2008:[249–251], 2007:[153–155,222],2006:[2,31,156,159,252] SatoY. 5 30 15 2020:[200],2018:[201,202],2012:[203],2006:[207] 2020:[230,234],2019:[48,236],2015:[238,253,254], StewartE. 11 70 16 2011:[19,244,249,250] 2019:[255],2018:[256],2016:[257,258],2014:[259], SudaY. 6 0 2 2012:[260] 2020:[200],2019:[261],2018:[201,202],2016:[257,258], TanimotoM. 8 30 17 2012:[203],2006:[207] 2015:[215],2012:[216],2010:[39,219],2009:[158], TobiasA.M. 6 578 10 2008:[251] 2016:[89,90],2014:[91,92,224,225],2013:[226],2011: TsunashimaH. 12 274 23 [204],2010:[262],2008:[205,206],2007:[26] WangP. 5 40 11 2020:[142],2019:[143],2018:[144,145],2017:[146] WangY. 6 40 11 2020:[142,263],2019:[143],2018:[144,145],2017:[146] WardC.P. 6 92 14 2013:[115],2012:[116],2011:[19,117],2010:[118,119] 2020:[230,234],2018:[264],2015:[238,239],2014: WestonP. 20 522 42 [24,242,243],2011:[19,244,245],2008:[249,250],2007: [153–155,222],2006:[156,159,252] YamanO. 6 41 11 2020:[174],2018:[65–67,175],2017:[69] 2020:[265],2019:[266],2018:[267],2017:[237],2016: ZhangW. 5 26 1 [12]

Researchonconditionmonitoringisconductedallovertheworld;however,several countriesarecharacterizedwithleadingroles.TheresearchconductedintheUnitedKing domischaracterizedbythemostoutstandingnumberofpublicationsandcitations,asis mentionedinTable3(moreover,ascanbeobservedinFigure12,theUKischaracterized bythehigheststrengthofinterests,asillustratedbyitshavingthehighestdiametercircle). ThisisnotsurprisinggiventhefacttheUKisthecountryoforiginofrailtransport.The nextcountries,assortedbythenumberofpublications,areChina,Japan,Italy,theUSA, theNetherlands,Sweden,andSpain—or,sortedbythenumberofcitations:Italy,Spain, theUSA,China,Japan,andSweden(theNetherlands,inthiscase,isplacedafterseveral othercountries).Onecanfindthereasonsforsuchanoccurrence:Inmostofthesecoun tries, highspeed train systems are developed, whose condition monitoring must be

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treatedwiththehighestinterest,forsafetyreasons.Itwouldbeinterestingtocompare thesedatawiththeresearchers’affiliationsaswell;however,affiliationsarenotanalyzed inthispaper.Thereasonforthisisthat,accordingtoVOSviewer’screators,theScopus databaseinformationonorganizationsmaynothavebeenharmonized—thatis,organi zationsmaynothaveaconsistentformat.

Figure12.Countries’specificcooperationintheresearchon“conditionmonitoring”.Source:Ownstudyappliedusing VOSviewersoftware(version1.6.15).

Table3.Countries’specificcooperationintheresearchon“conditionmonitoring”,obtainedusingVOSviewersoftware (version1.6.15).

TotalLink TotalLink Numberof Numberof Strength Numberof Numberof Strength Country Documents Citations Computedinthe Country Documents Citations Computedinthe Published Obtained VOSviewer Published Obtained VOSviewer Software Software United 164 2776 54 Finland 4 257 0 Kingdom China 71 447 35 Norway 4 13 2 Japan 45 387 8 Portugal 4 53 7 Italy 42 981 15 SouthKorea 4 199 0 United 41 755 10 Ireland 3 10 5 States Netherlands 33 171 11 Latvia 3 15 5 Sweden 32 258 11 Switzerland 3 6 5 Spain 25 938 20 Greece 2 1 0 India 22 97 6 Hungary 2 1 0 Poland 20 151 0 Iran 2 14 0 Australia 19 225 11 Mexico 2 16 1 France 18 192 8 Romania 2 0 0

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Turkey 14 141 2 Tunisia 2 18 2 UnitedArab HongKong 13 153 7 2 2 0 Emirates Germany 12 22 0 Indonesia 1 2 1 Canada 8 4 4 Malaysia 1 1 1 CzechRe 7 43 2 Myanmar 1 0 1 public Russian 7 58 3 Nigeria 1 3 0 Federation Singapore 7 21 0 Qatar 1 1 3 SaudiAra Austria 6 8 0 1 1 1 bia Denmark 6 20 2 Serbia 1 2 1 SouthAfrica 5 146 2 Taiwan 1 1 0 Belgium 4 64 6 Uganda 1 1 0

4.SystematicLiteratureReview Keywords are primarily the language constructs that link various publications. Therefore,specialattentionisgiventotheminthisreviewpaper.Allofthekeywordsfor 613 analyzed publications were grouped around the phrase “condition monitoring”. Therewere323individualkeywordsreportedintotal.Oneshouldbearinmindthatsome ofthesearepluralandsingularformsofthesameword(e.g.,“accelerometer”and“accel erometers”),whichwerenotunified,ascanbeobservedinFigure13.Fromtheviewpoint ofthisreviewpaper,developedespeciallyforaSpecialIssueofthejournal,selectedkey wordswerechosen.AllindividualkeywordswerecomparedwiththelistofSpecialIssue keywordsandthejournal’saimsandscope,accordingtothebestofourknowledge.Key wordsdirectlyconnectedtoinfrastructureandvehicles,aswellasvarioustechniquesand methods,wereexcludedfromthegraphicalanalysis.Wedecidedtoconsideronlythe keywordsconnectedtotheSpecialIssueofSensors.Thelistofselectedkeywordsisgiven inTable4;itwasdividedintothreeseparateparts:Thefirstpartisconnectedtovarious typesofsensorsusedindifferentsolutionsappliedtoconditionmonitoringinrailway transport.Thesecondpartpresentspublicationsconnectedtowirelessandonlinecom munication,whichleadsdirectlytothethirdpart,whichfocusesoncurrentaspectsof Industry4.0appliedtoconditionmonitoringinrailwaytransport.Anytimeakeyword giveninTable4ismentioned,itishenceforthnotedinboldinthispaper. Itisworthmentioning,despiteperhapsseemingobvious,thatdifferentkeywords characterize a given publication. Therefore, it is not possible to group all publications characterizedbyidenticalkeywordsinthetextwithin—forexample—asingleparagraph; thisisthepurposeofTable4.Itisthereforepossiblethatapublicationpresentedinagiven fragmentofthecurrentsectionconcernsadifferentgroupofkeywords.Inthecontentof thissection,agivenpublicationisdescribedonce,andanyreinstatedreferencingisrare; therefore,withinthescopeofthisdescription,morethanonekeywordmayoccurinthe immediatevicinity,whichwehopeshouldbeaconvenienceforthereader.Itshouldalso benotedthatsomepartsofthepapertreatrailvehicles’andrailtracks’conditionmoni toringseparately,whereasothersdotheopposite.Wehopethatthisdoesnotcauseundue interference.Moreover,wetriedtodescribethepresentedresearchbymaintainingthe chronologicalsequence(naturally,withminordeviations).

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Figure13.Completesetofkeywordssignificanttotheresearchgatheredaroundtheinternalkeywords“conditionmoni toring”.Source:OwnstudyappliedusingVOSviewersoftware(version1.6.15).

4.1.AnalysisofVariousTypesofSensorsUsedinDifferentSolutionsConnectedtoCondition MonitoringinRailwayTransport Asmentionedattheendoftheprevioussection,weinitiallypresentananalysisof selected papers connected to various types of sensors used in different solutions con nectedtoconditionmonitoringinrailwaytransportThelistofreviewpapers’specific keywordsstartswith“accelerationsensors”and“accelerometers”.Thesekeywordswere ascribedbytheauthorsinbothsingularandpluralform. InLiuetal.[198],twomeasuringsystems,namely,ElectronicSystemAnalysisFrog— Mobile,andavideogauge,wereusedfortheconditionmonitoringofrailwayturnouts. Theauthorsanalyzeddatameasuredwitha3Daccelerationsensormountedonthecross ing noseinorder toobtaindataonthethreedimensionaldynamicacceleration ofthe crossingnoseandtheverticaldisplacementofthesleeper.Theauthorsanalyzedrecord ingsfromcameras(installedoutsidethetrack)todetectthemovementsofthetargetsset ontherail(orsleeper)byanalyzingtherecordedvideos.DuringatestintheNetherlands, theresearchersobservedthattheverticalaccelerationofarailwaycrossingwasdramati callyreducedaftertampingand,simultaneously,thefrequencycompositionofthedy namicresponsesalsochanged.Severalyearslater,theauthorsLiuandMarkine[193]in vestigatedrailwaycrossingdegradation.Theauthorsassessedrailwaycrossingswiththe useofsensorbasedcrossinginstrumentation,andverifiedobtainedmeasurementswith theuseofthemultibodyvehicle–crossingmodel.Theauthors’resultsshowedthatthe huntingmotion(activatedbythetrackgeometrymisalignmentcoupledwithnonrecti linearityinfrontofthecrossing)ofthehighwheel–railimpactforcesduring’pass ingcausedprogressivecrossingdegradation.Adjacenttrackstructureswereanotherrea sonforcrossingdegradation,accordingtotheauthors’results. AccelerationsensorswerealsoappliedbyKänsäläetal.[268].Theauthorsfocused on the monitoring of rail track condition in Finland with use of sensor technology

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employingwirelessthreedimensionalacceleration(ascanbeobserved,variousresearch teamsusesimilarkeywords;however,withthedifferentdescription,asforthepreviously mentionedpaper,itwas“3Daccelerationsensor”).Thesystemwasinstalledonboard. Anomaliesduringrideswerecomparedtoknown,previouslymeasuredparameters,for exampleonbridgesandswitches.TheauthorsappliedmapmatchingandBayesianfilter ing,withtheaimofimprovingtheglobalpositioningsystem(GPS)locationaccuracyof themeasuredandcollecteddata.Theauthorsclaimedthatmeasurementscollectedby sensorsattachedtorollingstocksupportstrategicandoperativemaintenanceplanning. The next keyword connected to this subsection, namely “accelerometer(s)”, was foundinthecaseofthefollowingpublications. ThesystemdescribedinDanneskioldSamsøeandRamkowPedersen[269]consisted ofanarrayofaccelerometersthatwereappliedinordertomeasurethewheelprofile wear.Additionally,thiswasmeasuredwithlaserscannersandcamerasamanufacturer otherthanthatofthesystemofaccelerometers.Suchacombinationofdevicesensured diagnosisofdefectsofvarioustypes,suchaswheelflats,outofroundwheels,andcorru gatedwheels. InWestonetal.[156],theauthorsobservedselectedfeaturesinthebogieandaxle boxobtainedusingrobustsensors.Sensorssuchasaccelerometersandrategyroscopes weremountedontheinservicevehicle’sbogieandaxleboxes.Theauthorsclaimedin completenessofthetrackgeometryinformationobtainedviathesesensors.InWestonet al.[156],theauthorsappliedapitchrategyroscopemountedofabogieinordertoobserve meanverticaltrackgeometry,aswellasabogiemountedyawrategyroscopetoobserve meanlateralalignmentandwavelengthtrackcurvature.Theauthorsdescribedtheresults ofmeasurementscarriedoutontwoseparaterailwayvehiclesasameansofresearchval idation.Asidefrommeanverticalandlateralalignmentirregularities,themeasurements werealsoconnectedtocrosslevelandtwistfaults.TheauthorsofWestonetal.[153]men tionedthattheopticalsensorsthatwereusedinrailtrackconditionmonitoringsystems intheyearsoftheauthors’researchwereachallengingpartofthesystems,sincethese kindsofequipmentneedfrequentcleaning.Theauthorsalsoaddedthatasimplerand morecosteffectivealternativecouldbeusingasmallnumberofaccelerometersandrate gyroscopes.Theresultsoftheirresearchprovedthataccelerometersanddisplacement transducerscanprovidemoreaccurateresults.Atthesametime,apitchrategyroscope methodwasacclaimedasprovidingresultsoverawiderspeedrange,andtheuseofgy roscopeswasalsosupposedtobeasimplerandmorerobustarrangement.Verticaltrack irregularitieswereconsideredinWestonetal.[153],whereaslateraloneswerepresented inWestonetal.[154].Inthelatterpaper,theauthorspresentedmeasurements(lateral accelerationoryawrate)withsensorsmountedonabogieforestimationofthemeantrack alignment,withnoapplicationofopticalorcontactsensors.Theresultsshowedthata bogiemounted yawrate gyroscope can provide measurements sufficient to specify maintenance operations laterally; therefore, a sensing accelerometer is not required to monitorthelateralmotionofabogieinresponsetotrackalignment(however,theauthors claimedthiswasnotstraightforward). InLeeetal.[270],accelerometerswerefittedonhighspeedtrains’axleboxesand bogies.Measurementsofaccelerationinboththelateralandverticaldirectionswerecom paredtoacommercialtrackgeometrymeasurementsystem,whichenabledauthorsto obtainthefollowingconclusion:Themeasurementsobtainedfrombogiemountedaccel erometersprovidedmorereliableresultsthanmeasurementsfromtheaxleboxmounted accelerometersinthewavebandfrom70to150m.Thiswasunexpected,astheauthor mentioned,becauseaxleboxesfollowtracks’irregularitiesmorecloselythanabogieasa generalrule.This research wasannouncedin Lee etal.[271],whereatrackgeometry measurementsystemequippedwithalaser,acamera,andaccelerometerswasalsoused toobtaincommensuratemeasurementsforhighspeedtrains. TheauthorsofChellaswamyetal.[272]investigatedfuzzylogicfortheestimationof potentialirregularitiesintherailwaytrackduringtrainoperation.Theauthorselaborated

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afuzzytrackmonitoringsystem,whoseessentialpartswerevibrationsensorsmounted onanaxleboxandacarbogie,formeasuringaccelerationinboththelateralandvertical directions.Thesystemwasequippedtoautomaticallytrackthelocationofvibrationsand sendtheinformationtothecentraloffice;whentheGSMsignalwasinaccessible,thefuzzy trackmonitoringsystemautomaticallystoredthecoordinates.Theauthorsconcludedthat forverticaltrackirregularity,anaccelerometerplacedonabogiegivesbetterresultsthan oneonanaxleboxinaspeedrangefrom51to100km/h.Therefore,theirresultswere consistentwithLeeetal.[270].Chellaswamyetal.[273]describedasystem—namely,an intelligenttrackmonitoringsystem—thatcanbeadoptedtoeliminatethetrackirregular ities.TheauthorsdevelopedMEMStechnologyandplacedsensorsonbothanaxlebox andabogie—themeasurementswereobtainedinboththeverticalandlateraldirections. Aglobalpositioningsystem(GPS)wasappliedtodeterminethelocationwithinthesys tem;however,ifthesignalwasmissingthenanintelligenttrackmonitoringsystemauto maticallystoredthecoordinates.SimilarlytoChellaswamyetal.[272],theauthorsstated thataverticaltrack irregularityMEMSmountedonabogiegavebetterresultsthana MEMSmountedonanaxleboxforaspeedrangefrom51to100km. InBagshawe[274],theauthorcomparedservoaccelerometerswiththeaccelerome tersusedinMEMSsfortrackconditionmonitoring,withabogiemountedinstallation. Theauthorselectedaverticalprofileasthesubjectofacomparativetrialbetweenthe alternatives.Accordingtothepresentedresults,theMEMSaccelerometerofferedcompa rableperformanceatspeedsabove50mph(80.47km/h),anditwasespeciallyamatterof interestthatsolutionswiththeuseofMEMSsaretypicallylessexpensive;gyroscopes werealsoincludedinthesystem. Verticalandlateralacceleration(withuseofthreeaccelerometersignals)oftheaxle box,bogie,andcarbodywerealsothesourcesofmeasurementsinQinetal.[275];asin thecaseofseveralaforementionedresearchelaborations,thispaperfocusedonthedesign ofanonboarddevicefortrackfaultdiagnosisandrailirregularitydatacollection.The authorsproposedasystemforthecalculationoftrackfaultprobabilities. InYeoetal.[242],theconditionofthetrackgeometrywasassessedwiththeuseofa systemthatconsistedofthreeaccelerometers,threegyroscopes,atachometersignal,and aGPSfeedercomprisinganinertialmeasurementunit;thissystemwasmountedonthe bogie.Theaimoftheauthorswastoinvestigatetherenewal,degradation,andmainte nanceofrailtracks. ThesystempresentedinTsunashimaetal.[91]consistedofaccelerometers,arate gyroscope,aGPS,andamapmatchingalgorithmappliedinordertopinpointthelocation offaultsonthetracks.Thesystemallowedtheauthorstoestimatetrackirregularitiesfrom accelerationsintheverticalandlateraldirections,andalsotoestimatetherollrateofacar body.Inordertocollectthedatafortrackmaintenance,thesystemwasequippedwitha smartphonetosendthedatatoaserver.Trackirregularitiesforconventionalrailways werealsoestimatedbasedonaccelerationmeasurementsintheverticalandlateraldirec tionsandtherollrateofacarbodyinTsunashimaetal.[92];thissystem’sdevicewas developedtogivehigherperformanceforpracticaluse;therefore,thecompactonboard deviceswithGPSsystemswerefittedwithamapmatchingalgorithm,asinTsunashima etal.[91]andTsunashimaetal.[204].Theauthorsmentionedthattheirsystemprovides thepossibilityofthedetectionofrailcorrugationfromcabinnoiseviathecomputationof spectralpeaks.Additionally,asmartphonewasproposedasequipmenttosenddata. Tsunashimaetal.[224]developedasystemallowingfortheestimationofthetrack geometryandirregularitiesofShinkansentracksusingcarbodymotionsalone(AKalman filter was applied to solve the problem of not using other accelerometer placements). Theiraimwastoexaminetracktamping,withaviewtoanalyzingacceptablequalityof ridingcomfort.Theauthorsclaimedthatverticalirregularityestimationoftracksispos sible,withacceptableaccuracyforactualuse. Continuing description of the works under Japanese supervision, the authors of Tsunashimaetal.[89]andTsunashimaetal.[90]presentedanonboardsensingdevice

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equippedwithsensorsandaGPSsystem,anddiagnosissoftwarethatdetectstrackfaults basedontherootmeansquareofaccelerationobtainedfrommeasurementsinacarbody. Estimationoftrackirregularitybasedoncarbodyaccelerationwasappliedwiththeuse ofaKalmanfilter. InXuetal.[276],theauthorspresentedafaultidentificationmethodfordetermining thedynamicmanagementlevelsoftrackirregularitybasedonanevidentialreasoning rule(aclassificationmethodusinganovelcombinationruleasinXuetal.[277]);intheir research,theauthorsusedsampledatameasuredfromtwoaccelerometersmountedon anaxleboxandacarbodyofaninservicetrain. Milneetal.[278]focusedtheirresearchontheissueofcumulativedistributionfunc tions,andtheirutilitytoidentifytheatrestpositionintrackdeflectiondataobtainedfrom geophones(forvelocitymeasurements)oraccelerometers. Tešietal.[279]developedsoftwarethathelpstodiscoverandinterprettypicalsitu ationsconnectedtoirregularitiesonrailwaytracksbasedonmeasurementsfromfiveac celerometers(locatedonthetrack,body,andaxles).Chellaswamyetal.[280]developed a differential evolution algorithm in order to optimize the values of irregularities that werereceivedfrombogies’andcarbodies’accelerationmeasurementsbyMEMSsensors (accelerometers). These measurements gave the authors actual data about track align ment,whichwasfurtherinvestigatedwithuseofthemathematicalmodelandthefre quencyresponseanalysis. InSomaschinietal.[281],theauthorstookintoconsiderationtheconditionmonitor ingofinsulatedrailjoints,whichtodateweremonitoreddirectlybytrainedpersonnel. However,inpersoncontrolofarailwaylineistimeconsuming,andnoobjectiveparam eterscanguaranteethehealthconditionofajoint.Therefore,theauthorsdevelopeda systemconsistingofaccelerometersplacedontwowheelsetsofarailvehicle. Monitoringofthetrackconditionforlightrailtransport(trams)wasalsoofinterest toresearchers.InVinkóandBocz[282],theauthorsdescribedasystemwhosepurposeis torecordthedataoftriaxialwirelessaccelerometersattachedtothewheeldiscsofareg ular,inservicetram.Theauthorsmentionedthatsensorsmountedonaxleboxesandbo giesideframesarecommonlyusedworldwidetoidentifytrackirregularities.Atthesame time,theyprovedthatwheeldiscmountedsensorsprovidedamoreadequatesolution formonitoringlightrailtransporttracks.Ontheotherhand,metrotrackmaintenance was,todate,investigatedbyinpersoninspection,accordingtoKomalanandChauhan [283];theauthorsdevelopedawirelesssensornetwork(WSN)withsensornodesinclud ingaccelerometerslaidalongthetracks’length;intheiropinion,maintenancetimecanbe reducedthankstomeasurementsobtainedwiththeapplicationofsuchanetwork.This reductioncouldbeensuredthroughautomatedmonitoringbydetectingfaultsbeforees calation. Asidefromusingaccelerometersfortrackconditionmonitoring,somesignificantel ementsofrailvehiclesarealsoofinteresttoresearchers. AsystemforheavyhaulwagonswasproposedinLietal.[110]inordertobolster springfaultconditionmonitoring;theauthorsdesignedthesystemrelyingsolelyontwo accelerometers (excluding other measuring appliances) mounted on the front left and rightrearpartofeachcarbodyonaheavyhaultrain. Hajovskyetal.[284]developedalesstypicalconditionmonitoringsystem;theau thors’conceptfocusedontheimplementationofaprotectivefenceinstalledaboverailway tracks.Suchafenceensuressafetyagainsterodingrocksandothermaterialfallingfrom hillsidesnexttorailwaytracks.Thesemonitoringsystemsconsistedoffiberoptics,asde scribedinGuoetal.[285],andthesystemwasequippedwithaccelerometers(MEMS sensors),whereastheenergyforthesystemcamefromsolarpanels,assuchasystemcan beinstalledmoreeasilyinhardtoreachterrain. Sinceothersignificantelementsarementioned,itisworthaddinganotherkeyword here—namely,“acousticemissionsensors”—whichwastakenunderconsiderationinthe followingpublications:[165,166,168,286].

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In[166],theauthorsresearchedhighfrequencyacousticemissionpairwisevibration analysisofmeasuringdevicesinstalledonatraininordertoinvestigateaxlebearingde fectsandpotentialwheelflats.Acousticemissionsensorscoupledwithaccelerometers wereinstalledonanaxlebearingbox,withminimuminterventionwithinthevehicle’s equipment,asrequired. TheauthorsofPapaeliasetal.andHuangetal.[165,168]statedthathotboxwayside systemsaresubjectedtofalsealarms,andfaultyaxlebearingscanonlybedetectedshortly afterfailureandfastenoughtopreventtheireffects.Therefore,theauthorsdevelopedan acousticemissionsystem(withacousticemissionsensorsincluded)forthedetectionof axlebearingfaults.Suchasolutionwasbasedonthephysicaldependenceontheacoustic emissionsignal,whicharisesfromtheaxlebearingfault,resultinginnoiseandinterfer ence (ultrasound). The tests were conducted on actual freight wagons with artificially damagedaxlebearings.OnepaperFararooyandAllan[286]wasrelatedtothecondition monitoringanddiagnosticsofrailwaysignalingdevices;theauthorsemployedacoustic emissionandothertypesofsensors,withtheapplicationoftechniquesemployingneural networks. Bearingsusedinfreightservicearesubjectedtorollingcontactfatigue.Accordingto Tarawnehetal.[287],subsurfaceinclusionsinbearingsresultingfromsteelcontamination initiatesubsurfacefatiguecracks,whichpropagatecontinuously.Therefore,theauthors describedprognosticmodelsforspallsinitiatingonbearings’inner(cone)andouter(cup) rings,whichtogetherwithaharmonious,sensorbased,bearingconditionmonitoringal gorithmcanincreasesafetyintheuseofbearings. InSánchezetal.[288],theauthorsappliedtwoseparateapproachesinordertodetect faultsinrailwayaxles.Thefirstoftheapproacheswasconnectedtogatheringandanalysis ofaccelerationsignalsmeasuredinthelongitudinaldirection.Meanwhile,theauthorsap pliedthedatafusiontechniqueinthesecondapproach,whichledtoevaluationbasedon measurementsobtainedwiththeuseofsixaccelerometersbymergingtheindicatorcon ditionsaccordingtothesensorplacement.Thetotalnumberofconditionmonitoringin dicatorspervibrationsignalwasequalto54.Tofindthebestfeatures,theauthorsused themeandecreaseaccuracymethodofrandomforests.Theresultsshowedthatafusion ofapproachesand,consequently,indicatorsworkedefficientlytogetherinordertodetect acrackfaultintheanalyzedrailvehicles’parts. Conditionmonitoringconcernsvariousrailvehicles,passengerandfreightcars,and railinfrastructure—especiallytracks.Somestudieswerealsoconductedinordertomon itortheconditionofrailbridges,asinFaulknetetal.[263];theauthorsdevelopedarota tionalmeasurementsystem,whichoperatesbymeasuringquasistaticanddynamicmeas urementsofabridge,andwhichconsistsofaccelerometer(s)andgyroscopes.Gyroscopes weretreatedinthissystemascomplementarysensorsfortheinstrumentationoftherota tionalaccelerometer.Theauthorsusedsensorfusiontechniquestocombinethemeasure ments,whichresultedinanalysisofabridge’stiltingbehavior. Somestudieswerefocusedonfreightrailtransportonly(thepresentedsolutionsare connectedtocars;nevertheless,energyharnessingmayalsobeappliedforsensorsana lyzingtracks;therefore,itisalsoincludedinthisreviewpaper).TheauthorsofGaoetal. [142]notedthatelectricalwiresarenotprovided forfreightwagons;therefore, power monitoringsensorssuchaspressuresensorsandaccelerometerscannotbedirectlyap pliedforsuchvehicles’conditionmonitoring.Nevertheless,thesetypesofsensorsarees sentialforensuringoperationalsafety;therefore,theauthorsinvestigatedtheapproachof harnessingfreightwagons’vibrationalenergyinordertopowermonitoringsensors(a pressuresensorandatriaxialMEMSaccelerometer)withtheuseofanelectromagnetic vibrationalenergyharvesterwithaninertialpendulum.Thedevelopmentofrailtransport evokedadramaticincreaseinthemarketrequirementforrailsidemonitoringdevices andsensorsinChina,asmentionedinGaoetal.[146];theauthorsdevelopedaprototype forthesmartmonitoringofundergroundrailtransit;thepowersupplywouldbepro vided by local energy generation; as a local energy source, the authors considered an

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electromagneticenergyharvester.Powerforthesensornodesofawirelesssensornetwork (WSN)wasobtainedbyconnectinganaccelerometer,awindpressuresensor,andatem peraturesensor.ThementionedsensorswerecoordinatedbyaselfpoweredZigBeewire lesssensornode,whichwasalsodevelopedinGaoetal.[145]forotherpurposes,men tionedfurtherinthissection. AmongthepublicationslistedinTable4,nexttothekeyword“accelerometers”are [289]byFarkas—whoseauthorsreviewedmeasurementofrailwaytrackgeometry—and Chiaetal.[50],inwhichtheauthorsreviewedapplicationconsiderations,currentchal lenges,andprospectiveopportunitiesforimprovementsinrailroadtrackconditionmon itoringwiththeuseofinertialsensors(inertialmeasurementsensors)andGPSsignals. MuthukumarandNallathambi[49]verybrieflyreviewedselectedconditionmonitoring systemsandalgorithmsconnectedtothem—namely,automaticrailroadtrackinspection, andremotesensornetworksbasedonfuzzylogic.InYellaetal.[290],theauthors’review paperwasconnectedtoartificialintelligence.Anotherreviewpaper’sauthordescribed detailedinformationononboardconditionmonitoringdevices,withaparticularinterest intreatingthemassensornodes—namely,microelectromechanicalsensors,microproces sors,andtransceivers—creatingwirelesssensornetworksBernaletal.[47];theauthors alsoinvestigatedsystems,methods,andtechniques,focusingonsolutionsconnectedto applicationsforfreightwagonswithoutonboardelectricpower.Theyalsomentioned,as inFragaLamosetal.[54],thatsuchsystemsconnectedtotheInternetofThingsnetwork enableadvanceddataanalytics.TheauthorsofHamadacheetal.[48]reviewedmethods, techniques,andapplicationsconnectedtotheconditionmonitoringofrailroadswitches andcrossingsystems. Thefollowingkeywordswereassignedonlyonce:angularratesensor,contrastsen sor,eddycurrentsensor,andinfraredproximitysensor.Thepublicationsinwhicheach keywordwasassignedarementionedbelow. TheauthorsofSakamotoetal.[259]developedasystemconsistingofanangularrate sensormountedonabogieinordertomonitoranddetectflangeclimbthat occursforarailwayvehicleatlowspeeds. InLiuetal.[291],ahighspeedframegrabbersystemwasdesignedinordertomon itorbullettraintrackswiththeuseofimagerecognition.Thesystemincludedafastcam era,animageacquisitioncard,artificiallighting,acontrastsensor,andotherequipment. Resultsofthisresearchshowedthattheheightofcamerainstallationdidnotaffectthe accuracyofthedesignedsystem. Therailfastening,whichfixestherailstothesleepers,isoneofthecrucialcompo nentsinrailtracks.Itssignificantfunctionisconnectedtomaintainingthetrackgauge andstability.InChandranetal.[233],theauthorstookintoconsiderationthemonitoring ofrailfasteningconditionswiththeuseofaneddycurrentsensorsystemforfastener detection.Thiskindofsensorappliestheprincipleofelectromagneticinduction.Thesys temproposedbytheauthorswasabletodetectanindividualrailfasteningfromaheight of65mmabovetherail. TheauthorsofKuuttietal.[292]presentedtheirresearchontheenergyefficiencyof two escalators at different metro stations. Infraredproximitysensorbased pedestrian countingcanbeusedintheconditionmonitoringofescalators,accordingtotheauthors. Theauthorsalsomentionedthattheadjustmentofescalators’energysavingsettingscan beanalyzedbasedonperiodicalpedestrianpatterns(powerconsumptionwasalsoana lyzed). Theelectricsensingdeviceisthenextkeywordthatfocusestheinterestofagroupof researchers. OnepaperbyZhangetal.[293]broughtattentiontotheincreasingnumberofsensor nodescreatingsensornetworks;theauthorsobservedthatthetrackingandallocationof fiberBraggsensorsinelectricsensingdeviceswasbecomingabottleneckincondition monitoring;however,theirresearchwasnotdirectlyconnectedtorailtransport.

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TheauthorsofZhangetal.[294]developedasystemconsistingofafiberBraggsen sornetwork(consistingoffiberopticsensors,alsoknownasopticalfibers)dedicatedto trackconditionmonitoring,especiallyforhighspeedtrainsystems.Thisconditionmon itoringsystem(withanelectricsensingdevice)enablesmeasurementsofrailwaytrack temperature,displacement,andstrain.Theauthors’aimwastoshowthatthesystemwas adaptedtothecomplexnaturalenvironmentalterrainaroundrailways. InAnastasopoulosetal.[295],theauthorsinvestigatedtheabilitytoobtainsubmi crostrainaccuracywithfiberopticBragggratings,usingtheauthors’thennoveloptical signalprocessingalgorithmdesignedfortheidentificationofwavelengthshiftswithhigh accuracyandprecision.Thepaperwasnotdirectlyrelatedtorailtransport;however,it mentionsthatfiberopticsensorshadbeenusedfortherealtimemonitoringofrailway infrastructure.InRoverietal.[296],theauthorssuggestedafiberBraggsensorarraysys tem(electricsensingdevice)mountedalongarailwaytrack,forvariouspurposes;onthe onehand,theauthorsplannedthemonitoringofundergroundrailwaytraffic(suchasthe numberofaxles,thetrainspeed,andload)inrealtime,andontheotherhand,theiraim wastheconditionmonitoringofboththerailwaytrackandthetrainwheels,suchasthe estimationofrailandwheelwear. Meanwhile,inrelationtothepreviouskeyword,SongandSun[297]analyzedwheel defects.Theauthorsresearchedwheelpolygonsinparticular,withasystembasedonpi ezoelectricsensors(electricsensingdevices)insteadofstraingauges,whichhasthead vantagesofhighsensitivity,widefrequencyresponse,vastdynamicrange,andperfect electromagneticimmuneproperties. Afiberopticsensor,mentionedpreviouslybyZhangetal.(2017a)[294],isthenext setofkeywordsworthconsideration. AccordingtoIlieandStancalie[298],manyrailsystemsconsistofopticalfibercom municationsinfrastructure,especiallyinurbanareas,andthisiswhytheauthorsmen tionedthatitispossibletoapplyopticalfibersensorsforthestructuralhealthmonitoring ofrailways.InIlieandStancalie[298],theauthorsdecidedtoanalyzerailinfrastructure inordertoavoidbrokenrails,whichmighthaveseriousimplicationsforsafetyinrail transport.Theauthors’investigationsreferredtothemaintenanceoffiberopticsensors, andtheresponseofthistypeofdevicetotemperatureandstrainchanges,inorderto evaluaterailwayexpansion.Inthisparticularpaper,atestedmetalprobe—similartothe railmaterial—wasexposedtochangesintemperature. NaderiandMirabadi[299]mentionedthelimitationsandcapabilitiesofvarioussen sorsinrailwaytransport,whichresultedinthechoosingofasuitablefiberopticsensor fordetectingseveraldifferentparameters.Theauthorsproposedasensorforthepurpose ofmeasuringthetrain’sweightindifferentconditions.InEliaetal.[103],theauthors comparedfiberopticsmeasurementwithsignalsfromaccelerometers. InBuggyetal.[300],theauthorspresentedtheirpreliminaryresearchonthedynamic loadmonitoringof,stretcherbars,andswitchbladesonatramnetworkwith theuseofopticalfiberBragggratingsensors(alsoreferredtoasfiberopticsensors,optical fiberBragggratingstrainsensors,orstrainsensors).ArraysoffiberBragggratingsen sorswerealsostudiedinBuggyetal.[301]forthedynamicloadmonitoringoffishplates, switchblades,andstretcherbarsonatramnetwork.Accordingtotheauthors,themeas urementofstrainonacanprovideinformationonthetorqueofthebolts.Optical fiberBragggratingsensorswereappliedformonitoringchangesinthedynamicstrain signaturethatappearedintheabovementionedtrackelementsinresponsetothepassage ofatramoverthesensorlocation,aspresentedinBuggyetal.[302]. Freightrailairbrakesmightaffecttrainschedulinginanegativeway,whichiswhy theauthorsofPoddarandLipsett[303]decidedtoresearchconditionmonitoringtech niquesforairbrakeleakagedetection.Theauthorsworkedonqualitativeandquantitative evaluationandverificationoftheimplicationsofdetectingleaksultrasonicallyincold weatherconditions(accordingtotheScopusindexkeywordsthispaperwasconnectedto fiberopticsensors).

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NiandZhang[304]suggestedapproachesinBayesianmodelingfortheevaluation ofwheelquality.Theyappliedthismodelingtoatracksidemonitoringsystembasedon opticalfibersensorsinordertoobtaindataforrailbendingandtheconditionofwheels. AnopticalfibersensingnetworkwasalsodepictedinTametal.[305];theauthors developedaconditionmonitoringsystembasedonmachinelearninginordertodetect andidentifyvarioustypesoftrackdefects(railcorrugations,dippedweldjoints,andde fectsofrailcrossings). Facilitiesotherthanbogiesandtracksintherailwayvehicletrainneedcondition monitoring as well. Lin et al. [306] analyzed railway bridges; the authors assessed a skewed halfthrough plate girder railway bridge with the use of a fiberopticsensor based(fiberoptics)monitoringsysteminstalleddirectlyonabridgeduringitsconstruc tion.Theauthorsanalyzedandevaluatedtheimpactofaxleloaddistributionthroughthe evaluationoftrackballastspeciallyallocatedalongthebridgestructure.Meanwhile,the authorsofSengsriandKaewunruen[307]reviewedthepublicationsconnectedtobridge bearingsasamongthemostcommoncausesofbridgefailure.Thereviewwasconnected totheuseofsensorsandothersuchdevices. InNaetal.[308],theauthorsexaminedthecontactconditionoftheenergysupply system.Thiscontactwascertainlyassociatedwithapantographandanoverheadcontact line,andtheauthorsanalyzeditviaimageprocessingimplementedusingadevicein stalledonarailvehicle’sroof;thisdeviceincludedfiberopticsensors.InArthingtonet al.[309],theauthorsdescribedtheirimageprocessingapproach;theysuggestedatech niquebywhichtoextracttheverticalmovementofapantographfromimagestakenusing existingonboardcameras.Basedonthepostprocessingofimages,thecontactforcebe tweenthepantographandtheoverheadlinecanbeestimatedbasedontheobservedac celerationofthepantographhead.Inthissolution,Kalmanfiltersareprovidedtoelimi natemeasurementerrorsandsensornoise. Xuetal.[190]studiedprestressedconcretesleepersenrichedwithfiberopticsen sor–basedsystems,whichwerecalledselfsensingsleepers.Theauthorsconductedlabor atorytestsinordertodevelopanestimationofrailseatload,detectionofcracking,and identificationofballastsettlement—allwiththeuseofselfsensingsleepers.Theauthors concludedthataselfsensingsleepermaybedeployedonanoperationalrailwayinorder toprovidereliableandlongtermmeasurementsofrailaxleloadandballastpressure. MeasurementsobtainedinlaboratorytestsandbasedontheFEMsimulationsshowed thatthiskindofrailwayelementcanbeusedasarailseatloadsensorandaballastpres suresensorinactualapplicationontracks. OpticalfiberBragggratingstrainsensorsarethematicallyconnectedtofiberoptic sensors;therefore,somepublications’descriptionsaregivendirectlyinthefollowingfew paragraphs(withdisregardforanyalphabeticallistofkeywords). InHo[160],theauthorusedphotonicsensorswithfiberBragggratingarraysinor dertomeasurethetemperatureandstrainappliedinvariousrailwaytransportsolutions. Thesameyear,theresearchteaminHoetal.[161]presentedasolutionthatappliedfiber Bragggratingsensorarraysfortraindetection,trainweightmeasurement,anddatacol lectionforcharacteristicsofwheel–railinteractioninordertoenableraildefectdetection. InWeietal.[37],theauthorsdescribedarealtimesystemthatwasdesignedtomon itorwheeldefectsbasedonfiberBraggsensors.Thissystemensurestheminimizationof theriskofwheeldeteriorationbymonitoringtypicaltrainwheeldefects,whichaccording toNielsenandJohansson,Attivissimoetal.,JohanssonandNielsen[310–312]includede fectssuchaswheelflats,outofroundness,spalling,andshelling.Theadvantageofthe systemdescribedinWeietal.[37]isthatsensorsandconnectingfibersinstallednextto therailroadside(electricsensingdevice)arepassivetoelectromagneticinterferencefrom therailroadenvironment.Therefore,thiskindofsystemdoesnotneedanyadditional powersources,andcanbeinstalledatsignificantdistancesfromthemeasuringpoints. ThesystemcanbeintegratedwithotherapplicationsofrailroadfiberBragggratingsen sors,suchasthetrainaxlecountingpresentedinLieetal.andWeietal.[313,314],for

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example.Thesensorswereinstallednearthefootoftherails,whichwasconsideredtobe themostsensitiveandpracticalallocationinWeietal.[314];therefore,thesystemmight beabitofanuisance,especiallysincetherailsmustbepolished,andpairsofsensorsmust beinstalledinthesamelocationtoensurethatwheelslocatedonbothsidesofanaxle passthetwomeasurementpointsatthesameinstant.Thissystemconsistsofrealtime cameras and RFID receivers installed for train identification, and results in mapping. Moreover,inAttavissimoetal.[311]—mentionedpreviously—theauthorsusedalaser diodeandachargecoupleddevicecameraformeasurementsofwheelsandrailheadpro files.Theconglomerationofsuchadeviceallowedustoevaluatethewheel–railinterac tionquality.Thissystemrecognizesanomaliesinwheel–railcontact;however,according toWeietal.[37],itdoesnotidentifythetypeofwheelorraildefect.Furthermore,systems withdevicessuchaslasersandcamerasrequireprecisionsetupandcalibration,andcan notinterferewiththeextremetrack gaugeorthevehiclegauge,posingchallengesfor practicalrailroadapplication. Alemietal.[315]presentedtheirresearchonthemeasurementofwheeldiameteras apartofrailvehicles’conditionmonitoring,withtheuseofacommercialmonitoringsys temcalledtheWheelImpactLoadDetector(WILD).Thissystemcollectedmonitoring datainordertodetectpotentialdefectsinwheelsbasedonthemeasurementsofatleast onestrainsensor,withtheabilitytomonitormorefeaturesofthewheelconditionusing thesamesystem.Thissystemwasalsousedbyotherresearchers,asinGaoetal.and Harrisonetal.[146,316],formorecomplexsolutions. The next keyword, “gyroscope”, was mentioned previously in [50,91,153,154,156,242,263,274,289];someotherpublicationsarebrieflydescribedinthe nextparagraph. Tsunashimaetal.[262]consideredasystemthatcandetectcracks,corrugation,and otherirregularitiesintracks,especiallyforbullettrainsthatarethepartoftheShinkansen railroadsystem;thissystemiscalledRAIDARSS3,andcanalsobeusedinconventional trains.Withintheauthors’system,inservicevehiclesareequippedwithamicrophone, accelerometers,arategyroscope,aGPSreceiverforpositionaldetection,acomputerfor analysis, andananalogdevicetransferringsignalsfromeachsensortothe mentioned computer—thissystemwasalsomentionedpreviouslyinTsunashimaetal.[205].Allof theequipmentwasputinsideacompact,portablesensorboxinstalledinsidethecarbody. AstheauthorsofTsunashimaetal.andKojimaetal.[262,317,318]claimed,asetthatcon sistsofsimplesensorsandGPSissufficienttoserveasaprobeinordertomonitor,detect, andanalyzeavehicleduringitsoperation.Furthermore,suchasystemcandetectmainte nanceproblemsatanearlystageoftheiroccurrenceHayashietal.[319].Accordingto Tsunashimaetal.[262],thedetectionofrailcorrugationwithdirectmeasurementsignals (inparticularthresholdprocessing)isproblematic;therefore,inthissystemtheauthors proposedusingamicrophonefordetectingcorrugationwithspectralpeakcalculation. Spectralpeakcalculationwasappliedtodetectrailcorrugationbasedonmeasurements obtainedfromcabinnoise.Corrugationfindingsweretransformedwithintheanalyzing computeronthebasisofmultiresolutionanalysis(MRA),afterDaubechies[320],andFFT analysis.Theauthorsdidnotseeanycontraindicationswhenusingaccelerometerstode tecttrackirregularities(inthesystem,trackirregularitieswereestimatedbasedonincom ingsignalsfromtheverticalandlateralaccelerationofthecarbody).AsTsunashimaetal. [262]mentioned,trackirregularitiesareestimatedfromtheverticalandlateralaccelera tionofthecarbodyinthissystem.Agyroscopeisusedinordertodistinguishlineirreg ularitiesfromlevelirregularities.Theresultsofusingthissysteminthefieldtest(during theresearchtheRAIDARSS3systemwasinstalledonsixN700seriesbullettrains)were comparedwiththoseobtainedbymultipleinspectiontrainscollectivelyknownas”Dr. Yellow”,whichrunontheTkaidandSanyShinkansenlines,alongwithanotherin spectiontrain”Easti”,whichrunsontheThoku,Jetsu,Nagano,Yamagata,andAkita Shinkansenlines,andcontributestoShinkansentracks’safety.AGPSwithinthesystem and a mapmatching algorithm are used to locate faults on tracks. As mentioned, the

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systemdescribedinTsunashimaetal.[262]cananalyzeirregularities,cracks,andcorru gation;signalsofdifferentfrequenciesindicatewhatkindoftrackfailure(ofthosethree mentioned)hasbeendetected. Thenextkeywordis“heterogeneoussensors”,whichoccurredinonlyonepublica tion.Fumeoetal.[321]composedaconditionmonitoringsystemconsistingofthreemod ules:adatafusionapplication,asystemintegrator,andavisualizationsystem;thelatter wasreferredtoasthehuman–machineinterface.Thesystemwasdevelopedinorderto operateseveralheterogeneoussensorsintherailwaysystem.Inthistime,evenmoresys temswithvarioussensorsweredeveloped. Wherethekeyword“inertialsensors”isconsidered,itwasmentionedonlyinChia etal.[50]sofar. Wardetal.[118]reviewedadvancedconditionmonitoringsolutionsforrailvehicle bogies,andinvestigatedvehiclebasedsensorsformonitoringtherunninggear,aswellas suspensionandwheelparameters.Thework,inthiscase,reliedonsimulations,utilizing modelbased condition monitoring and Kalman filters with accelerometer data. The methodalsorequiredknowledgeofthetrackirregularitiesaswellasaccelerometerson allbodiesforthebestresults.TheauthorsofWardetal.[116]observedthatwheel–rail contactcausessignificantdisruptiontotheoperationoftherailwaynetwork,especially duringtheautumn.TheauthorsdevelopedamethodincludingaKalman–Bucyfilterin ordertoobtainanestimationofcreepforcesinawheel–railcontactarea.Thealgorithm gathereddatawiththeuseofinertialsensorsmountedonthevehiclebogieandwheelsets. Creep forces were also considered in Ward et al. [117]; the authors reported dynamic methodofthedetectionofcreepforceswithinertialsensorsinstalledunderneatharail vehicle.InWardetal.[19],theauthorsbroadlydiscussedtheuseofinertialsensorsin stalledonrollingstockinordertomonitortrackandvehicleconditions;theauthorsre calledtheappropriateallocationofsensorsfortrack/vehicleconditionmonitoring.More over,theauthorsdiscussedstandardsofdatasentinsuchasystem.Asidefrompotential monitoringopportunitieswiththesuggestedsystem,theauthorsprovidedcertainoper ationalinformationforuseonboardatraininrealtime. Bhardwajetal.[322]researchedasystemwithinertialsensorsonboardservicetrains forthedetectionandlocalizationofrailroadtrackirregularities.Insuchasystem,noise andreducedsignalstrengthwereadditionallygeneratedbythegeospatialpositionofthe GPSreceiversandthenonuniformsamplingoftheinertialsensors.Therefore,interfer enceinthemeasuredsignalsdecreased,leadingtohigherratesoffalsepositivesandfalse negatives.Theauthorsdevelopedamethodtolimittheoccurrenceofsuchasignalto noiseratiodecrease. MeiandDing[45]tookintoconsiderationamethodthatreliedoncrosscorrelations betweenheterogeneousmeasurementsfrominertialsensorsmountedonarailbogie.The authorsanalyzedverticalprimarysuspensionsofbogies;nevertheless,theyclaimedthat themethodmightbeappliedinordertoobservepotentialfaultsinthelateraldirection onprimarysuspensions,aswellasinboththeverticalandlateraldirectionsonsecondary suspensions.Theauthorsalsomentionedthatusingthismethodandsystemenablescon ditionmonitoringinotherdynamicsystemscharacterizedbysymmetricalconfigurations. InVanrajDhamiandPabla[323],theauthorsclaimedthatvibrationanalysisviasta tisticalmeasureswasoftenusedforthediagnosisofgeardefects;however,acousticsig nals—intheopinionoftheauthors—werealsocharacterizedbyhugepotential.Theau thorsdidnotkeepawayfromstatisticalanalysesbasedonthemeasurementsofvibration sensors.Moreover,theycombinedvibrationalandacousticsignals into afusionofvi broacousticsignals(sensordatafusion,sensorfusion)thatproducedmorepromising resultsthanvibrationaloracousticfeaturesindividually. Laser scanning was also among the interests of researchers for railway vehicles (trackaccompanying),aswellasforthetracksthemselves,asismentionedbelow. Kimetal.[245]presentedtheirresearchonsleepers’conditionmonitoringanddis placementinparticularlycriticalareas.Thesystemcollecteddataviatheuseofdevices

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mountedonavehicle,includinginertialsensors,lasersensorsmountedonabogie,po sitionalsensitivitydevicesensors,andgeophonesmountedonthesleeperandconnected viaacontrollerareanetwork(CAN)bus.Thisbusallowsthetransmittingofdatabetween multiplesensornodesinthesensornetwork.Theauthorsplannedtohandleupto16 sleepersinacriticalzoneusingjustonesetoftheaforementioneddevices. InReitereretal.[324],theauthorsstatedthatlaserscanning,asafastandhighpre cisionmethod,couldbecomeanapprovedoptionforthemeasurementoftracks,overhead lines,clearanceprofiles,androllingstock.Withtheuseofsuchamethod,itwaspossible toobtainmeasurementsfromuptoseveralmillionpointspersecond(highdensitiesof measurementsspots),whileatthesametimeitwaspossibletoobtainprecisemeasure ment accuracy. The authors described an overview of laserbased sensor systems for measuringrailwayinfrastructure,andhighlightedadvantagesanddrawbacksassociated withsuchsystems. Theconditionofarailwaywheelcanbeanalyzedbasedonwheelprofilemeasure mentwiththeuseofautomaticsystemsmountedalongarailwaytrack.Oneofsucha system’sadvantagesispinpointingbadwheelsatanearlystage.Nevertheless,measure mentdatainsuchasystemhavetobecontrolled;therefore,theauthorsofAsplundetal. [93]developedanassessmentmethodforthevalidationofindividualwheelprofilemeas urements,inordertoprovideadequateaccuracyofthewheelprofilemeasurementdata. Wheelmeasurementswerecomparedbyfoursensors—onepersideoftworails;laser scanningwastakenintoconsiderationinthisresearch.InAsplundetal.[94],theauthors presentedawheeldefectdetectorasadevicewithwhichtomeasureandmonitorwheels’ parametersalongthetracksoastodetectfaultywheels,whichareoneofthereasonsfor trackdamage.Wheeldefectdetectorscancatchfailuressuchasflats,largeshelling,and severewheelpolygonization. IntheTkkaidShinkansenline,theaccelerationonthecarbodytrainshasbeen measuredcontinuouslysincetheinaugurationoftheTkaidShinkansenin1964(Araiet al.[325]),inadditiontotrackmeasurementconductedusingtrackgeometrycars.In[326], theauthorAraipresentedatwodimensionallaserdisplacementsensorandmeasuring devicethatcompensatesforthecarbodyaccelerationinordertoallowthemeasurement ofallparametersoftrackirregularityintheShinkansensystem’stracks. TheauthorsofMcCormicketal.[327]examinedtheinteriorconditionofrailwaytun nelswiththeuseofimagingandlaserscanningbasedtechniques,whichledtorapiddata capture.Thesemethodsensuredthereductionoreliminationofthenecessityfortrack accessandvisualinspections. Thefollowingkeywordsarecharacterizedbyasmallnumberofpublications,yetare neverthelessofqualitativevalue.Theseare:“multiplesensors”,“onboardsensors”(this keywordispresentedinSection4.2,sincethethematicscopeofthatsectionsuitsitbetter), “pressure and temperature sensors”, “rechargeable sensor nodes”, “strain gauge sen sors”,and“remotesensors”(alreadypresented). Aperiodicoutofroundwheelgeneratesexcitationfrequenciesevolvingintotheres onanceofatrackandvehicle;polygonaldefectdetectionwasusedinAlemietal.[328]in ordertostudythementionedaspect.Differentpolygonaldefectsweresuccessivelyrec ognizedwiththatsolution,usingmultiplesensors,andthedominantharmonicswere decomposed. OnepaperbySomàetal.[329]presentedexperimentaldatameasuredviatheappli cationofanonboardunitinstalledonanintermodalfreighttrain.Thedevicewascom posed of various kinds of sensors to describe the actual conditions of a monitored wagon—especiallyabrakingsystem.Thesensorsusedinthismonitoringsystemwereas follows:pressureandtemperaturesensorstoenableanalysisofthebrakes’condition, andanaccelerometerforanalysisofthewagondynamics,installedonthewagon’schas sis. Asystemofwirelessrechargeablesensornodesfortheanalysisofcorrugationprob lems in urban railways was developed in Lu et al. [143]; this system included

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electromagneticinductionprinciplebasedenergygeneratorsandwirelesssensornodes, creatingawirelesssensorsnetwork.Thesystemwaselaboratedforthepredictionofthe dynamicresponseofrailwaytracksbrandedbyrailcorrugation. Thenextinlineisanexceptionalkeyword:simply“sensor(s)”,theirderivatives,and relatedkeywords.Asmaybesuspected,manypublicationsweredefinedbyoneofthese terms—allofthesepublicationsarementionedinTable4,whilesomeofthemarealso givenbelow.Therefore,thepublicationsmentioneduptotheendofthecurrentsubsec tionareonlyafewexamples,anddonotexhaustthefullrangeoftheissue. TheauthorsofBrunietal.[26]surveyedtheapplicationofsensors,electronics,and computerprocessingtechnologiestosuspensionsandrunninggear;theauthorsfocused onthecomplementaryissuesofcontrolandmonitoring. InWardetal.[119],theauthorsresearchedtherecursiveleastsquaresalgorithm;a solution based on linear estimations was suggested because of the observation of the strongdiscontinuousnessofthecontactgeometryduringwheel–raildisplacement.The algorithmofwheel–railcontactgeometryinarailvehiclesystemwasdiscussedbasedon asystemequippedwithmultiplesensorsandhighcapacitycommunicationbuses,both withhighprocessingcapability. BöhmandDoegen[330]deemedswitchestobeacriticalpartofrailinfrastructure; theystatedthatswitcheswereexposedtostrongforcesduringtrainoperationandinci dentallyextremeweatherconditions.Usinghugequantitiesofsensorswouldbefartoo expensive;therefore,theydevelopedasystemtomonitortheconditionwithouttheap plicationofsensors.Theauthorsnotedthatexternaldatasourceshadtobeidentifiedand accordingly integrated in order to ensure the necessary information otherwise gained fromsensorswasobtained,asrailwayswitchandcrossingsystemsareexposedtoharsh workingconditionsthatmakethemsusceptibletofailuresandbreakdowns;therefore, theseareelementsofintensifiedconditionmonitoring.Furthermore,theauthorsofHam adacheetal.[114]consideredamodelbasedfaultdetectionmethodwithamodifiedre sidualbasedtechniqueappliedtoelectromechanicalrailwayswitchsystems. InHenaoetal.[105],theauthorsavoidedtheuseofmechanicalsensorsinthediag nosisofmechanicalfaultsinrailwaytractionsystems;theydemonstratedthenoninvasive techniqueofusingatractionmotorasatorquesensorthroughitselectromagnetictorque estimationfortorsionalvibrationmonitoring(alsopresentedinKiaetal.[107]).Kiaetal. [106]presentedthissolutionpreviouslybasedonresearchusingadeviceonareduced scale, in which the experimental setup of the traction part was developed and instru mentedbasedontheextrapolationofrealscaleparameters.Electromagnetictorqueesti mationwiththeuseofnoninvasivemeasurementasaresourceformechanicaltorsional stressmonitoringinaninductionmachinedrivingsystematnonstationaryoperatingcon ditionswasalsopresentedinKiaetal.[108].Testsandvalidationoftheproposedmethod wereconductedwithasquirrelcageinductionmachineconnectedtoaonestagegearbox. Kiaetal.[109]claimedthatacomplexstructureandthediagnosisoftheelectromechanical systembasedonnoninvasiveelectricalsensorsdrewspecialattentionfromrailwaytrac tionresearchersinrecentyears.InKiaetal.[109],arealtractionbogiewasusedforme chanicalconditionmonitoring. Anelectricpointmachineisadeviceforoperatingrailwayturnouts,especiallyfrom aparticulardistance.Theconditionmonitoringofsuchadevicewasreviewedanddis cussedinOyebandeandRenfrew[52,53].Anapproximatemethodandacomputersimu lationtoreplicatemaladjustmentwiththeuseofnoninvasivesensorswereproposedby theauthors.Zhouetal.[331]proposedtheconditionmonitoringofanelectricpointma chine with approximately 15 separate sensors, either connected directly to the electric pointmachine,ornexttothetracksurroundingit. Railwaytracksrelyonnumerousboltedjointconnectionsappliedtoensurethesafe andreliableoperationofboththetrackandtracksidedevices.Similarlytootherelements ofrailwaysystems,manualmaintenanceofboltedjointsischaracterizedashighcost,dis ruptive,andunreliable,andcanbesubjecttohumanerror.Therefore,theauthorsofTesfa

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etal.[100]developedasystemthatautomaticallymeasuredtheclampingforceofeach individualboltedjoint;asaresultoftheiranalyses,piezoresistivesensorsusingtheforce attenuationmethodwerechosenasthemostproductivesensorsforthepurposesofthis research.Thestraingaugeandcapacitorsensorsallowedthemtoobtainfewersatisfying results(thecomparisonanalyseswereperformedusingdesignoptimizationtechniques). InTsunashimaandMori[38],theauthorsadaptedtheinteractingmultiplemodel methodtodetectfaultsinrunningrailwayvehicles;theyusedthevehiclemodel/system fornumericalanalysesofthegivensolutions,includingthelateralandyawmotionsin thecaseoftherailvehicle’swheelsetsandbogie,andthelateralmotioninthecaseofthe vehicle’sbody.Theauthorsdecidedtousesensorsformeasuringthelateralacceleration andyawrateofabogie,andthelateralaccelerationofabody. InLalletal.[147,148],theauthorsmentionedthatavehicle’sconditionmaybemon itoredagainstfailureswiththeuseofresistancespectroscopybasedstatespacevectors, rateofchange,andacceleration—thelattertwoforthestatevariable.Theauthorsmen tionedthatactualprognostichealthmanagementapplicationareasincludefatiguecrack damageinmechanicalstructures—suchasthoseinrailstructures—mostlywithvarious typesofsensors. InNgigietal.[6],theauthorsappraisedtheuptodateconditionmonitoringtech niquesappliedforrailwayvehicledynamicsbyanalyzingvariousmethods’advantages and disadvantages. The authors identified wheelset condition monitoring, and track basedsensorsfortheeffectiveconditionmonitoringofvehicles.Theyconsideredthechal lengeoffindingpropermeasurementtechnologies,alongwithrelevantandcorrectpa rametersthatcanbemeasuredtomeettheexpectedaimsofsuchsystems. TheexperimentspresentedinCzechyraandFirlik[132]werecarriedoutvianumer icalsimulationsfordifferenttramtypes,conductedwithconsiderationofselectedfaults, undervariousrunparameters—suchasspeed,vehicleload,tracktype,etc.Thesystem developedbytheauthorstookintoconsiderationanalysesofmetricssuchassuspension condition,wheelsurfacewear,ridecomfortandstability,andsafetyagainstderailment. Thesystemwasequippedwithacommunicationmodule,apositioningdevice,andaset ofsensors.Eachmeasurementofsensorslocatedonthevehiclewascomparedtothenom inalvaluesandhistoricalmeasuresfromthesamesensor(comparisonofaccelerations andpositionsignalsmeasuredduringrunsoverthesametracksection). TheauthorsofThakkaretal.[332]presentedaprototypesystemforpredetectionof wheeldefectsusingrailmountedsensors;theauthorsconcludedthattheirsystemmight beusedfortheanalysisofwheeldefectsbyusingeithertrackmountedorwheelmounted sensors,afterselectedcalibrations. InMatsumotoetal.[203],theauthorsdevelopedathennewmethodofmeasuring wheel–railcontactforces,whichwasbasedonnoncontactgapsensorsplacedonpartsof thebogieinwhichnorotatingpartsaresituated.Themethodandthesystemwereverified onthesubway/metroline.Theauthorsobtainedsomeimportantcharacteristicsofwheel– railcontactmechanics(i.e.,derailmentcoefficients,fluctuationsofthecoefficientsbeing similaronstraightandcurvedtracks)duringthesystemexploitation,withmutualinflu ence. ChudzikiewiczandStelmach[333]describedacomplexsolutionformonitoringthe technicalconditionofavehicleandtrackatthesametime;thesystemdescribedbythe authorswasintendedtoconsistofanetworkofsensorsmountedonaselectedpartofa railvehicle.Thedataacquisitionunitandadataserver—bothenrichedbysoftwarededi catedtotheanalysisandmanagementofdiagnosticdata—werealsoincludedwithinthe system.Thedata—i.e.,accelerationsignalsobtainedduringrunsovercriticalpartsofthe tracksection(e.g.,crossings)—wereintendedtobecomparedwithoneanother. Limitationinsensorquantitiesisimportant,forexamplewithrespecttocostsorthe sendingofsignals.Kirkwoodetal.[334]discussedsuchatopic,withreferencetorailway transportaswell;intheopinionoftheauthors,addingsensorsisnotadifficulttask;how ever,associatedcostsareconnectedtothosesensors—suchasthecostofeachsensor,the

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costofutilitiesrequiredtopowerthem,thecostofdatagatheringandprocessing,andthe costofstorageofthatdata.Thesecostsmustbetakenintoconsideration—especiallycur rently,whenwirelesssensornetworksareofhugeinterestforvariouspurposes. InGroosetal.[335],theauthorspresentedaquasicontinuousconditionmonitoring systemthatwasdevelopedtoanalyzeshortwavelength(fromafewcentimeterstoafew meterslong)defectsofrailwaytracks,suchasrailcorrugation.Themeasurementswere obtainedinthisresearchusingavehicleembeddedsensorasatriaxialaxleboxaccelerom eter.Theaccelerationsensordatawerecombinedwithadigitalmapoflinearrailwayin frastructureandothercrucialoperationaldata,whichwereimplementedwithinaland sidedatamanagementsystem.Trackselectivegeoreferencingbymultisensorfusion,the axle box acceleration data applied for pattern recognition, and further intelligent data analysiswereusedforthetemporalconditionoftrackanalyses. Ningetal.[336]tookintoconsiderationtheproblemofsmallamplitudehuntingde tectionbeforetheoccurrenceoflateralinstabilityobservedinthecaseofhighspeedtrains. Dataobtainedwithonlyonesensorwereconsideredbyauthorstobeinsufficient;there fore,inordertoimprovetheaccuracyandrobustnessofthemonitoringsystemforthe detectionofsmallamplitudehunting,theauthorsdevelopedamultisensorfusionframe workusingtheDempster–Shafertheory.Thismultisensorfusionframeworkconsistsof severalsteps,describedinthepaperindetail.Theauthorssuggestthatthementioned frameworkcanalsobeusedasagearboxmonitoringsystem,sincenonstationarysignals areacquired. InXuetal.[264],theauthorsproposedamethodofwheelconditionmonitoringin thecontextoftheirwear;themethodwasdevelopedinordertoprovideanalysesforhigh speedtrains’wheelwearusingonboardvibrationsensors.Accordingtotheauthors,tech niquesandstatisticalmethodsforsignalprocessingwereappliedinordertoobtainmul tilocationvibrationdataandestimatethewheelwear. Rothetal.[337]suggestedasystemfortrackconditionmonitoringdevelopedviathe sensorfusion(sensordatafusion)ofaglobalnavigationsatellitesystem(GNSS)andin ertialmeasurementunit(IMU)measurements,togetherwitharailwaynetworkmap.This systemischaracterizedbyanofflinepositioningproblem;therefore,theauthorsproposed positioningestimationbyatwostageapproachbasedonGNSSdataandarailwaymap. TheseapproachstagesaredeterminationpathhypothesesfromtheGNSSpositionsatthe firststage,whereasthesecondstageisconnectedtosmoothingextensiontotheKalman filter,i.e.,nonlinearRauch–Tung–Striebelsmoothedpositionandspeedestimation.The measurementsofrailwaytrackcoordinateswiththeuseoftheGNSStechniquewerealso appliedinWilketal.[338];theauthorsassessedthequalityofobtainedmeasurements, andinvestigatedthepotentialfortheirfurtherprocessing.Theiraimwastodevelopthe usageofgeometricallyconstrainedparametersofthebasevector,alongwithspecialized digitalfilteringappliedwithinnumericalanalysesconnectedtotheresearch,inorderto preciselydeterminethetrackaxis. Wolfetal.[17]suggestedaconceptofatramconditionmonitoringsysteminwhich ameshedandwirelesssensornetworkcanprocurevibrationmeasurementdata.The conceptwasbasedontheassumptionthatsensornodeswereeventtriggered,inanen ergyefficientmanner.Thesystemwouldevaluatedecentralizeddatacollectedbymeshed andautonomousvibrationsensornodes. AnunmannedhighspeedvehiclewasstudiedinGong[339];theauthorpresented ananomalydetectionmethodwiththeuseofanaccelerationsensoronanaxlebox.This method’sauthorproposedtheuseofaquasidoublelayerchloroprenerubberfilter,in steadofanalgorithmicsolution,toisolatethehighfrequencycomponentoftheshock accelerationsignal.Apartfromthevibrationaccelerationofanaxleboxofavehicle,the deviceaccompanyingthismethodconsistsofadataacquisitioncardandatrainspeed sensor.Theauthordescribedchangesinthestatisticsofparticularparametersconnected toconditionmonitoringindetail.Theauthorconcluded,forexample,thatdifferencesin vibrationsuppresscharacteristicsofanaxleboxdamper,andthattheuseofresponseratio

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todetecttheabnormalityoftheaxleboxdampercandeterminetheabnormalcondition oftheaxlebox’sacceleration(detectionoffault)oftheunmannedvehicle. InBarmanandHarazika[340],theauthorsattemptedtodevelopasensorbasedsys temtoachievetrainconditionmonitoring;thissystemwasavibrationsensor,together withtheinsignificanteffectoftemperatureonitsmeasurements.Inordertostorevibra tiondata,anembeddedsystemusingtheArduinoUnoboardwasdeveloped.Theauthors observedthatthetimedomainanalysisprovidedinformationontheslipandderailment tendenciesofatrain,whilefrequencydomainanalysisindicatedtheconditionofvarious traincomponents.

4.2.AnalysisofVariousTypesofSensorsUsedinDifferentSolutionsConnectedtoWirelessand OnlineCommunication AsmentionedinthefinalpartofSection3,ananalysisofselectedpapersrelatedto wireless and online communication used in different solutions connected to condition monitoringinrailwaytransportispresentednext.Thelistspecifickeywordsforthese papers is given in Table 4, starting with “Bluetooth communication”, through “GPS”, “mapmatching algorithm” (already presented in the previous subsection), “smartphones”,andplentyofsimilarkeywordsconnectedtowirelesscommunications and wireless networks—such as “wireless sensor network”, “wireless sensor network (WSN)”,“wirelesssensornetworks(WSNs)”,“wirelesssensornode”—aswellas“artifi cialintelligence”and“bigdata”. InDaadbinetal.[341],theauthorspresentedaconditionmonitoringsysteminwhich componentsallowthemonitoringofbothaxlevibrationandpantographload.Themoni toringsystemconsistedmainlyofstraingaugeinstrumentation,telemetrysystems,and dataloggers.Themeasureddata—suchastorqueandbendingmomentsonagearbox shaft,andloadoractualaccelerationonapantograph—werecontinuouslymonitoredand diagnosedduringthetrains’routineoperations.Rapidandaccurateidentificationofthe locationandexacttimeanddateofanalarmingeventwereprovidedbyBluetoothradio communicationwithtrainsandtheironboard GPSsystems.Thissystem wasalso de scribedinapreviouspaperbyDaadbinandRosinski[342](especiallyinthecontextofa numericalmodelforthetractionsystem),anditwasmentionedthatitusedtwosubsys tems—namely,adigitalprocessingmodule,andareceivingsignalandtransmitterunit, bothinstalledinsidethecarriage. ThedevicedesignedandpresentedinMizunoetal.[343]consistedofaGPSreceiver, anaccelerometer,andananalog–digitalconverter.Asensorwassettledonavehiclefloor, asopposedtoaxlesorbogiesasinthecaseofnumerousotherstudies.Theauthorsrecog nizedsuchalocationasbeingeffectiveforcapturingverticaltrackdisplacements,andthe measurementofaccelerationresponseonavehicle’sfloorasapromisingindextocapture railwaytrackdisordersintheverticaldirection.Theauthors’aimwastotracearouteand, atthesametime,collecttheaccelerationresponseofapassengervehicle. InLoSchiavo[344],theauthorspresentedawireless,energeticallyautonomoussen sornetworkfortheonboardmonitoringofrailwayfreightwagons.Theenergyharvester usedinthissolutionensuredindependentnetworksofsensornodesforeachwagon.Re ductioninenergyconsumptionwithoutworseningqualityofservicewasassuredbythe useofabiperiodiccommunicationschemeforthelocalwirelesstransmission:aGPSre ceiver,andaGPRStransceiver;theauthorcalledthis“optimizedmanagementofthesleep modes”. InAimaretal.[345],theauthorspresentedaconditionmonitoringsystemforanin termodalfreightwagon.Thesystemwaspoweredbyabattery,asmostfreightcarsare notpoweredbytractionsourcesofelectrification.Theauthorsmentionedthatthemain aimatthebeginningoftheirresearchwastoenrichadatabaseforthedevelopmentofa diagnosticalgorithmforan embeddedremotemonitoring systembasedonmonitored parameters,whichwere:GPSposition,pressuresofthepneumaticcircuitofabraking

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system,thetemperatureofabrakehangerandblocks,andonboardaccelerationrecorded duringtrainoperation. Zoccalietal.[346]assessedridecomfortinsiderailvehiclesaccordingtoISO2631. Forthispurpose,theauthorscomputedverticalfrequencyweightedaccelerationforsub sectionsoffixedlengths,andcommensuratelymappedtheobtainedmeasurementson geographicinformationsystems(dataweregatheredonrailwaylinescharacterizedby automatic train operation). This enabled the authors to assess whether discomfort oc curredduetolocalizedirregularitiesorwornrailswitches.Thesystemtheauthorsused consistedofaninertialmeasurementunit(IMU)integratedtogetherwithaGPSreceiver. InordertoenabletheuseofsmarttechnologieswithGPSintheconditionmonitoring offreightrailcars,aneffectivepowersourceisrequired.InPanetal.[347],theauthors developed a freight railcar electromagnetic energy harvester at the secondary suspen sions,withamechanicalmotionrectifiermechanism.Theharvesterwasdesigned,mod eled,andtestedinbothlaboratoryandfieldconditions.Thepaperpresentsthedesign, modeling,inlabtests,andonboardfieldtestsoftheenergyharvester.Accordingtothe authors,thisenergyharvestingdevicecanacquirethevibrationalenergythatisusually dissipatedorwasted. Davies[348]mentionedthattherailindustrywasunwillingtorespondtotheintro ductionofsmartphonesandtabletcomputers,whichhaschangedrecently.InAungetal. [349],theauthorsconsideredonboardsensormeasurementandsatelliteimageanalysis appliedfortheearlydetectionofpotentialdeteriorationoftrackcondition.Theauthors usedanaccelerometerinasmartphone(onboardsensor)inordertomeasureacceleration intheverticalandlateraldirections.Thesmartphonewasplacedagainstthecarbody;the authorsassumedthatasmartphonevibrateswhenpassingirregularrailtracksections. In Seraj et al. [350], the authors worked on a system called RoVi, which was a smartphonebasedframeworkforthecontinuousmonitoringofnumerousconditionin dicators for infrastructure conditions and assets. The system used data acquired via smartphoneGPSandtheinertialsensorsofdevicescarriedbytrainpassengerstopro vide actual spacetime information on monitored infrastructure (the crowdsensing as sumptionswereapplied).Thedatawereprocessedbyanoptimizedprocessingalgorithm, whichledtocomputingindicatorsconnectedtorailroadtrackgeometryfeaturessuchas a,atwist,acurvature,andanalignmentfordifferentsegmentlengths,aswellasa pathroughnessindex. BridgelallandTolliver[351]noticed,aswithmanyotherresearchers,thenecessityof lowercostandmorescalablesolutionsforsensorsusedinonboardrailvehiclemonitoring approaches.Thesesolutionsaregenerallyassociatedwithagreatdealofuncertainty,es peciallyconsideringthatvariousaspectsofanomaliesthatoccur—suchaspotholes,frost heaves,swelling,andcracking—canrapidlyprogresswithanincreaseindynamicvehicle loadingandchangesinweathercycles[351].Therefore,researchersusedsmartphonesas devicesthatmightbeusefulinordertoevaluateavarietyofmethodsandapplications usedinrailvehicles.Theauthorsunderlineddemandingaspectsconnectedtogeospatial positioningsystemreceivers.Theexperimentsconductedinthisresearchfoundthatthe longitudinalGPSerrorspreadrangedfrom21mto32m(orsometimesevenexceeded thisvalue),whiletheerrorinlateraldirectionspreadrangedfrom9mto15m.Moreover, accordingtotheauthors,theerrorconnectedtogeodesicdistanceincreasedlinearlywith traversalspeed,from16matspeedsof18.8m/sto27mathighwayspeedsof31.3m/s. Theauthorsopenedthedoorforfuturedeliberationsonusingsuchsystems’positioning. Twotypesofspecialequipmentvehiclesarementionedintheliterature:oneofthem isthetrackrecordingvehicle(TRV),whichisusedfortrackinspectionsinordertomain tainthesafetyofthetracks,whiletheotheristhehauledtrackrecordingcoach(TRC), whichcollectstrackgeometrydata,Chudzikiewiczetal.andWestonetal.[176,238].In Balouchietal.[352],theauthorsclaimedthatintheUK,TRVswereused;however,they developedalowcostTracksuretrackmonitoringsystem.Thissystemisdedicatedtothe detectionoftrackvoidsandothertrackdefectsandirregularities(Balouchietal.[352]

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claimedthatthesystemcanalsodeterminecorrugationandwheelflats);itmakesuseof theexistingonboardGSMRcabradiooperatedineverysingleUKtrain,throughthefit tingofasensorcardthatdetectstrackconditionoverthreeaxesoftrainvibrations.Iden tificationofvoidsisrealizedbyaccelerationsignalsmeasuredwiththeuseofsensorcards installedwithinthevehiclecabsofmultipletrainstoassesseachtracksection.Inthissys tem,themachinereadsdatapacketscomputingtheaccelerationsignal’srootmeansquare (RMS).Additionally,thesystemsetstheRMSthresholdingdynamically,basedonma chinelearningfromthemultitrainanalysis;thedataareobtainedusingaGPSsystem. InBosomworthetal.[112],theauthorsconsideredthesituationsinwhichaparticular parametercannotbemeasureddirectlyonaspotconnectedtoit—suchasrollingcontact, forexample.Atthesametime,theauthorstookintoconsiderationthefactthatadevice operates in extreme environmental conditions. They stated that simulation modeling playsasignificantroleinsuchcircumstances;therefore,theauthorsintegratedamulti bodysimulatorintoanonboardpassengervehicledevicewithlocalsensors,suchasGPS, inordertosendaninputsignaltoasimulatorthatcomputedindicatorsofwheel–rail contactandderailmentrisk. In Bogue [353], the author briefly mentioned the application of a wirelessbased stressmonitoringsystemthatwasdedicatedtothemonitoringofrailwaytracksandroll ingstock;however,otherlargestructureswerealsomentionedinthispaper. Hodgeetal.[32]surveyedwirelesssensornetwork(WSN)technologiesforrailway transportconditionmonitoring.Theauthorsanalyzedsystems,structures,vehicles,and machineryinthecontextofwirelesscommunication—theywereespeciallyinterestedin solutionsforthemonitoringofinfrastructure(bridges,railtracks,trackbeds,andtrack equipment)alongwithvehiclehealthmonitoring(chassis,bogies,wheels,andrailcars). InKrichenetal.[354],theauthorsresearchedopticalwirelesssensornetworkarchi tecturebasedonvisiblelightcommunicationandfreespaceopticstechnologies.Inorder toensurerailtransporttrafficsafety,lightbasedsensornodeswereproposed.Suchsensor nodesallowedtheauthorstomeasuretheamplitudeandthefrequencyoftherailvibra tionduringatrain’soperation.Opticalsensorslocatedattherailfootusedvisiblelight communicationtotransmitappropriatedata.Freespaceopticalcommunicationwasused toenablecontactbetweengateways,andenabledcommunicationbetweenopticalsen sors. InMaddisonandSmith[355],theauthorspresentedasolutionthatsupportswireless sensornetworks—especiallyforrailtunnels,trackbeds,andtrackmonitoring—duringits constructionandfutureexploitation.Thissolutionwasdescribedinthespecificcontext oftunneldeformationmonitoringduringengineeringworks.Moreover,thesystemen suredtheabilitytomeasurefluctuationsintrackcantandtwist,aswellasthelongitudinal rateofchange.Thesemeasurementswerecollectedwithextremelyhighprecisionand stability,whichtheresearchersattributedtosensornodes.Thewirelesssolutionisbe lievedtobesuperiortothosebasedonopticalsensors.Theauthorsmentionedthatthe measurementsanddataobtainedwerehighlystableandrepeatable,withoutanyneedfor frequentmaintenanceorconfigurationofreflectors,withoutsusceptibilitytonoise,and— sincewirelesssensornetworkswereused—thesystemavoidstheneedforwiring.This batterypoweredsystem(batterylifeextendedtoover10years)ensuredhighprecision, MaddisonandSmith[356]. Theauthorsof[23]reviewedvarioustypesofwiredsensorsandsensorsinwireless sensornetworks(WSN)thatwereusedfortheconditionmonitoringofrailwaytracks. Theauthorsdescribedtheapplicationofparticularsensorsinordertomonitortheperfor manceofdifferenttrackcomponents,especiallyduringunpredictedoccurrences. InNsabagwaetal.[357],theauthorspresentedaconditionmonitoringandreporting structure,whichreliesonawirelesssensornetworkwithsensordatatoestablishauto maticweatherstationhealth.Thiskindofsolutionmightalsobeusedaspartofatransport system;therefore,itisalsomentionedinthispaper.Basedontheidentifiedproblemsina database,SMSoremailwassenttotheconcernedperson.

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InHusselsetal.[358],theauthorsresearchedaconditionmonitoringsystemthat revealstracethreatsinpipeswithfiberoptics(opticalfibers);however,theauthorsmen tionedthatthegreatpotentialfordistributedacousticandvibrationalsensingwasalso rapidlyevolvingintrainandtrackmonitoring,e.g.,inLienhartetal.[359].Thesubjectof optoelectronicandphotonicsensorsusedfortheconditionmonitoringofpipelineswas consideredinLopezetal.[360].InLienhartetal.[359],theauthorsreportedcertainap proachestocontinuouslymonitoringrailwaytracksandvehiclesbyusingdistributedfi beropticsensingtechniques.Themonitoringsystemconsistedoffiberopticstrainsens ingcablesthatwerelocatedalongrailwaytracks,andstrainchangesduetoraildefor mationsweredepictedbydistributedBrillouinmeasurements.Suchmeasurementsallow thedetectionofpossibledamagetotherailwayinfrastructureduetonaturalcausessuch asmudflow,avalanches,floods,andlandslides,andaccordingtotheauthorscanalso headoffsecondarydamagebyenablingfastandappropriatecounteractions.Theauthors usedopticalcommunicationcablesthathadalreadybeenallocatedalongrailinfrastruc turetodetectflatspotsinrailwaywheels.Insummary,theauthorsmonitoredbothinci dentsanddeformations. Wirelesssensornetworksdonotrequireregularreplacementofbatteries.InNaram panaweetal.[361],theauthorsfoundanenergyharvestertocollectenergyfromcurrent carryingcablesavailablewithinrailwaysystemsandbuildingfacilities.Theauthorsex ploredanultrathininductivelycoupledtransformerthatmaybeusedforenergycollec tionpurposestopowerupsensornodes.Theauthorsdescribedthatthissolutionmaybe manufacturedonaflexibleprintedcircuitboardintheformofanautonomoussensor node. ThesesystemsinGaoetal.[142,144,145]arecalledselfpoweredwirelesssensornet worksforwirelesscommunication.Theconditionmonitoringsystembasedonenergy harvestingwasdescribedintheabovementionedpaperGaoetal.[142].Atrackside,vi brationbased,electromagneticenergyharvestingprototypewaspreviouslyanalyzedin Gaoetal.[144];however,thissystemwasdesignedfortrackanalysisonly.Thesystem describedinGaoetal.[144]providedpowertorailsidemonitoringdevicesconnectedto sensorsthatensuredthecollectionofwheel–railvibrationenergyduringthetrainrides. Thisprototypewasequippedwithwirelesssensornodescoupledwithanaccelerometer andatemperature/humiditysensor.InGaoetal.[145],theauthorsextendanenergyhar vestingsystemwithaselfpoweredZigBeewirelesssensornodeasacoordinatorofan accelerometer,atemperaturesensor,ahumiditysensor,andaninfrareddetector. EnergyharvestingwasalsoofinterestinEspeandMathisen[362];theauthorseval uatedtheimplementationabilityofenergyconnectedtothemagneticfields,inorderto increase the lifetime of distributed conditionmonitoring systems. This energy was in tendedtobegainednearelectrifiedrailwaytracks.Accordingtotheauthors,itmaybe usedforlowpoweroperationandlowdutycyclewirelesscommunicationinthecaseof conditionmonitoringsystems. InCiietal.[363],theauthorspresentedasystemfortheconditionmonitoringand structuraldiagnosisoffreightrailwayvehicles.Theauthorspresentedthedesign,andthe laboratorytestresultsofwirelesssensornodes(WSNs)equippedwiththreeenergyin dependentandautonomousonboardproductionsystemsbasedonsolar,wind,andme chanicalvibrationpoweravailableduringtrains’operation.Thethreesensors,usingvar iousenergysources,werecomparedaccordingtotheirfatiguebehaviorandperformances duringlaboratorytests.

4.3.CurrentAspectsofIndustry4.0AppliedinConditionMonitoringinRailwayTransport AsnotedinthefinalpartofSection3,ananalysisofselectedpapersconnectedto selectedaspectsofIndustry4.0appliedinconditionmonitoringinrailwaytransportis presentedbelow.Thelistofthesereviewpapers’specifickeywordsgiveninTable4starts with“artificialintelligence”and“bigdata”,proceedingto“Industry4.0”,andthrough

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“intelligentconditionmonitoring”,“intelligentsystems”,“InternetofThings”—andalso “InternetofThings(IoT)”—andfinisheswith“robotics”. Numerousabovementionedpublicationspresentedextensiveconditionmonitoring, signalgathering,andsignalcollection;nevertheless,theapplicationofdeeplearningand artificial intelligence approaches applied to the detection, diagnosis, and prediction of faultswaslimited. Yellaetal.[290]summarizedtheresearchandoutcomesofalargenumberofstudies connectedtoartificialintelligencetechniquesappliedtotheautomaticinterpretationof datafromnondestructivetestingcarriedoutonrailinspectionproblems;certainly,the authors’reviewwaspertinentatthattime. AspecificsubjectofresearchwastakenintoconsiderationinWuetal.[364];theau thorsanalyzedthedynamicchangesinvibrationradiationnoiseinabullettrain’spassen gercompartmentluggagerack.Accordingtotheauthors,athreedimensionalspectrum is sensitive to changes in train operation status, operation speed, and track condition; therefore,theirmethodcanbeextendedtonoisemeasurementandanalysisofthepassen gercompartment.Theauthorsmentionedthatthismethodcanbeusedforthemeasure mentandanalysisofbothpassengercompartmentnoiseandexteriornoise.Moreover,the authorssuggestedthatthismethodcanbeappliedfortrainoperationstatemonitoring andcomfortableevaluationofpassengercompartmentnoise. TheauthorsofSaeedetal.[365]discussedaspectsofconditionmonitoringandfault diagnosisinaFrancisturbine.Theauthorsintegratednumericalmodelingwithseveral differentartificialneuralnetworktechniques(artificialintelligence).Thisresearchwas notdirectlyconnectedtorailtransportconditionmonitoring;nevertheless,itwasindexed inthedatabaseunderthematterofinterest,anditcanbeapplied,especiallyconsidering thattheauthorsmentionedthatthemultipleadaptiveneurofuzzyinferencesystemstech niquewasappliedformonitoringanddiagnosisindifferentpracticeareas,aswellasfor railwaytrackcircuitsinChenetal.[366]. InMarwala[367],theauthordescribedartificialintelligencemethods;amongthese methods,theauthorincludedneuralnetworks,particleswarmoptimization,genetical gorithms,andsimulatedannealing. TheauthorsofSuetal.[127] developedlineardynamic,hybridmodel predictive control,whichwasusedtodescribetheevolutionofrailwaytracksegment’scondition. Theauthorsofthismodelprovidedadetailedproceduretoillustratehowtotransform themodelbasedoptimizationproblemintoastandardmixedintegerquadraticprogram mingproblem.Forfuturework,theauthorsplannedtoenrichthismodelwithparameters obtained from track measurements. According to the database, this research was con nectedtoartificialintelligence. Anotherstudyconnectedtotheuseofartificialintelligencewasconnectedtother malcameraimageprocessing.InYamanandKarakose[175],theauthorsanalyzedthe detectionofrailsurfacedefectsbasedoncomplexfuzzyautomata.Theauthorsapplied Otsu’smethodforimagestakenfromthermalcameras.Theobtainedresultswereusedas theinputvaluesofthecomplexfuzzyautomatasystem.Accordingtotheauthors,the proposedmethodwasnotaffectedbyenvironmentalconditionswhenusingathermal camera.Theirresearch’ssuccessratiointhedetectionofrailsurfacedefectswasalmost 90%. InSunetal.[368],theauthorspresentedanapproachforthedetectionofrailjoint failureonarailwaytrackbasedonaccelerationmeasurementsbytrainingconvolutional neuralnetworks(artificialintelligence).Thesystemdetectedjointsorfailures,andwas abletodetectfailuresonbothrailsusingonemodel.Thedevelopedsystemworkedwith rawdatainputinordertoobtainareductionofheavydatapreprocessing.Theexperi mental results showed that both convolutional neural networks—namely, ResNet and FCN—obtainedsatisfactoryperformance. InMoslehetal.[369],theauthorspresentedanapproachthatallowstheidentifica tionofwheeldefectsandwheelflatswiththeuseofawaysidemonitoringsystem.Todo

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so,amethodologyofenvelopespectrumanalysiswasapplied(artificialintelligencein cluded,accordingtothedatabase).Theauthorspresentedtests,anddiscussedandana lyzedthesensitivityoftheproposedapproachtotrackirregularities,randomizationof boththepositionsandsizeofawheelflat,andperturbationresultingfromdifferentnoise intensities. Finketal.[5]providedathoroughassessmentofactualdevelopments,drivers,chal lenges,solutions,andplannedstudiesinthefieldofdeeplearningandartificialintelli genceapproachesappliedtoprognosticsandhealthmanagement(PHM)applications, takingrailtransportintoconsiderationaswell.Theauthors,forexample,mentionedpub licationsofGibertetal.[370]thatdescribedtheuseofimagedatacollectedinorderto buildamultitaskdeepconvolutionalneuralnetworkforrailwayinfrastructuremonitor ing.Theelaboratedconvolutionalneuralnetwork(CNN)wasabletorecognizeelements ofrailwayinfrastructure.Moreover,theapplicationoftheCNNallowedthedetectionof damage,suchascrackedconcretetiesandmissingorbrokenrailfasteners. Nieblingetal.[371],stayinginthemainstreamoflowcostsensorapplications—e.g., KnightPercivaletal.[372]—oninservicetrains,suggesteddeeplearning(artificialintel ligence)inordertoanalyzelargevolumesofsensors’measurementdata.Theauthors combinedcommonsignalprocessingmethodsanddeepconvolutionalautoencodersand clusteringalgorithms.Suchacombinationofmethodsandtechniqueswasappliedtofind variousrailtrackanomaliesandtheirpatterns;furtherimprovementsanddevelopments wereplanned. InNúñezetal.[126],theauthorsobservedthatcountlessrailwaytrackcondition monitoringdata(terabytesofdatafromvarioussensors)werebeingcollected,yetnot fullyused,becauseofinsufficientsuitabletechniquestoextracttherelevanteventsand crucialinformation.TheauthorsdiscussedconditionsofthefiveVsofbigdatainrailway conditionmonitoringsystems,namely:volume,velocity,variety,veracity,andvalue.The authorsdescribedanaxleboxaccelerationmeasurementsystemcalledtheABAsystem, whichincorporatesfailureandmaintenanceinformationinordertobeadaptiveandself learning. InLinetal.[257,258],theauthorssuggestedapplyingbigdataanalysesinthecondi tionmonitoringofrailwaysystems,sincethesemonitoringsystemsenablebigdatatobe accumulated during commercial operations. In this research, the authors use a naive Bayesclassifierasamethodofbigdataanalytics,whichhelpedtoidentifyindividualities ofrailwayvehicles. InLinetal.[256],theauthorsdescribedanontrackconditionmonitoringsystemto measurethewheelloadandforceofrailwayvehiclesgeneratedinthelateraldirection,in ordertoobtainbothvehicleandtrackinspectioninformationafterbigdataprocessing. LinandSuda[255]developedamethodimplementedintheontrackconditionmon itoringsystemthatassistsinthedetectionofavehicle’sabnormalitybyaddingbothve hiclesandtrackinspectioninformationwithinbigdataprocessing.Thismethodwasim plementedinthemonitoringsystemforthewheelloadofrailwayvehicles,andforforce generatedinthelateraldirection.Theauthors’resultsaidinthedeterminationofwhether ornotvehicleandtrackinspectionwereexecuted. InGrossonietal.[373],theauthorsanalyzedtrackstiffnessasakeyphysicalparam eterfortrackqualityassessmentandlongtermtrackdeterioration.Theauthorsverified theroleoftrackstiffnessandaspectsofitsspatialvariabilitythroughasetofexperiments basedondigitalinformationandbigdata.Theauthors’analyseswereakindofsensitivity assessment,withothervehicleandtrackphysicalparametersbeingunderquantitative alteration(thefollowingparameterswerevaried:vehicleunsprungmass,speed,andtrack verticalirregularities;dataconnectedtostiffnessandverticalirregularitywereanalyzed usingasampleprocedurebasedonanARIMAstochasticprocess).Trackgeometrydete riorationrateswereestimatedbasedonavehicle–trackinteractionmodel,whichwascou pledwithseriesofloglinearregressionmodelsappliedtoanalyzetheimpactofvariable

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parametersontrackdeterioration.Accordingtotheauthors,spatialvariabilityoftrack stiffnesswascharacterizedashavingasignificantimpactontrackdeteriorationrates. Pistoneetal.[374]discussedtwovariedseriesofresultsofacontinuousmonitoring systemoftheoperatingtrack:firstly,formassivetrainwheels,andsecondlybyimpulse testsperformedinnineselectedstationsoftheundergroundinVienna.Thereasonforthe testwasthesubstitutionofconventionalresilientwheelsformassivewheelsinorderto increasetheperformanceofvehicles,reduceoperationalcosts,andincreasesafety.Se lectedtrainscirculatingintheundergroundnetworkwereequippedwithmassivewheels, andmodulesofthesystemwerefittedwithinertialsensors.Ninemeasuringsystems wereinstalledwithinthenetworkintheformofmonitoringdevicesappliedtocontinu ously record data. Radio frequency identification tags (Internet of Things employed) wereinstalledfortrainidentification,andtheinformationonvehicles’condition—and especiallydataontheimpactofmassivewheels—weresenttoheadquarters.Threeyears ofthistestingallowedanalysisofthebigdatausedforrealtimediagnosticsofinfrastruc ture. InAnetal.[9],theauthorsresearchedarecurrentneuralnetworkwithalongshort termmemoryinordertodiagnosefaultsinmotordrivesystemscharacterizedbybigdata measurements.Theauthorswantedtoexcludethelimitationsofexistingintelligentmeth ods—namely,thefactthatthetrainingdataandtestingdataaresubjecttothesamework ingconditionsinsuchmethods.Theydevelopedanovelthreelayermodelinspiredbya recurrent neural network coupled with transfer learning, and verified the proposed methodusingarollingelementbearingdataset.Theresearchers’resultswereanimprove mentincomparisontoartificialneuralnetworksandsparsefiltering.Theauthors’meth odsensuredrapidjudgmentoftheconditionoftheanalyzedelements. InXuetal.[375],theauthorsdefinedabnormalsegmentsinconditionmonitoring thatnotonlyreducethequalityofdataforconditionmonitoringandbigdataanalysis, butalsobringaheavycomputationload.Theauthorsmentionedthatthesesegmentsare missingsegmentsanddriftsegments,whichoccurunavoidablyinbigdataanalyseswhen sensorsundergofailures,networksegmenttransmissionisdelayed,orthereisaccidental lossofsomecollecteddata,etc.Therefore,theauthorselaboratedanabnormalsegment detectionmethodinordertoimprovethequalityofbigdata.Thesolutionwasverified with actual wind turbine data; however, this method might be used for the condition monitoringofrailtransportmachinery. InXuetal.[376],theauthorsdevelopedanoverviewofpredictivemaintenancesys tems,withaparticularemphasisonexistingmodels,methods,andalgorithmsensuring datadrivenfaultdiagnosticsandprognostics.Theauthorselaboratedsuchanoverview becauseofthemassivegrowthintheresearchofconditionmonitoringdataofindustrial equipment(industrialbigdata).Whenrailtransportwasconsidered,theauthorsmen tioneddeBruinetal.[377]asastudyonalongshorttermmemoryrecurrentneuralnet workthatwasappliedtolearnfromdependenciesonrawrailwaytrackcircuitdataand theirdiagnosedfaults. InSakietal.[378],theauthorssuggestedadiscontinuanceofonboarddatacompu tingwiththeuseofsignalprocessingcoupledwithenrichedmachinelearningbasedap proaches.Theauthors’reasonforsuchadecisionwastolimitthetransmissionbandwidth onthepublicmobilenetworks.Theresearchersdevelopedadeviceconsistingoftwomain parts:thefirstpartwasadataclassificationmodelthatclassifiedInternetofThings(IoT) dataintomaintenancecriticaldataandmaintenancenoncriticaldata,whilethesecond partwasadatatransmissionunitensuringcommunicationmethodstotransmitbigdata torailwaycontrolcenters.Forthetransmissionofmaintenancenoncriticaldata,theau thorsproposedaspecificmethodthatemploystrainstationsaspointsatwhichdataare transferredanddeliveredtomentionedcenters.Asthissolutionconcernedpassengers’ stations,itcouldthereforebeappliedonlyinthecaseofpassengertrains. Parhietal.[379]developedaninferencesystemofmultipleadaptiveneurofuzzy characteristics, and a methodology related to the detection of cracks in structures.

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Accordingtotheauthors,thismethodologycanbeappliedfortheconditionmonitoring ofdynamicstructures,suchasrailwaystructures;nevertheless,theauthorsdidnotcon ductresearchonrailstructuresdirectly.TheauthorsmentionedthattheauthorsofChen etal.[366]studiedfaultdetectionanddiagnosisforrailwaytrackcircuitswiththeuseof neurofuzzysystems. InMolodovaetal.[129],theauthorspresentedtheresultsofrailjointconditionmon itoringbasedonaxleboxaccelerationmeasurements.Theauthorsconsideredmeasure mentsobtainedfromtrailingandleadingwheels,inthecaseofboththerightandleftrails. Theobtainedcharacteristicsaimtoobtainknowledgeofthequalityandthestateofthe insulatedjointinordertodevelopintelligentpreventivemaintenancestrategies(intelli gentsystems).Theauthorsvalidatedtheirmonitoringmethodbyinpersonvisualtrack inspectionsandhammertestsoftheinsulatedjoints. Avehiclebornebatteryisoneofthepiecesofequipmentusedinmagneticlevitation trains,whoseconditionhighlyaffectstheefficiencyofthetrain’soperation.InZhanget al.[267],theauthorsdevelopedasensornetworkdesignedtomonitorbatteries’condition ineachcar—especiallythebatteries’temperatureandremainingcapacity.Thissolutionis basedontheInternetofThingsconcept,thankstowhichatrainoperationcontrolsystem collectsmeasurementsofallcarsinamaglevtrainbyusingitscommunicationnetwork throughtheradiocommunicationsystem.Furthermore,inGaoetal.[145],theauthors benefittedfromthedevelopmentoftheInternetofThingsintheareaofintelligentrailway transport. InSramotaetal.[380],theauthorsreferredtoanautonomous,nearrealtimesystem fortheconditionmonitoringofrailwaytracks,points,andcrossings;thesystemworks duringtheoperationofanearbytrain.Sensorsorsensornodescreatedwirelesssensor networks.Astheauthorsmentioned,theaggregationofdatawasrealizedwithalow power,widearea,InternetofThingsnetworkstructure.Suchanetworkprovidedpost processingbigdata. Karadumanetal.[65]presentedamethodbasedonimagestakenfromthecameras monitoringtherailways.Theseimagesweretransmittedtoacentralcomputerusingthe conceptoftheInternetofthings(IoT)asatechnologycontinuouslyappliedforcondition monitoring systems (including artificial intelligence). The authors applied image pro cessingtechniquestotheseimages,andafterwardsmeaningfuldatawereextractedinthe centralcomputer.Thissystemwasdevelopedforcontactlessfaultdiagnosis—especially forpantographsandcatenary.Itshouldbementionedherethat,althoughthisreviewpa perismostlyconcernedwithconventionalvehiclesandtracksinrailwaytransport,the othersolutionsandtheirconditionmonitoringaregettingtoamorerapidstage.Onesuch solutionistheconceptofmagneticlevitation(popularlycalledmaglev)coupledwithso lutionsstandardizedfortheconceptofIndustry4.0,makingthepairanexampleofintel ligenttransportation.InSunetal.[381],theauthorsdevelopedamethodforthecontrol ofsuspensionformedium–lowspeedmaglevtrains.TheauthorsincorporatedtheInter netofThings(IoT)technologyandanadaptivefuzzycontroller(developedbytheau thors).Firstly,theauthorsestablishedamathematicalmodelfortheanalyzedsuspension systemofthementionedtrains,andthentheycomposedtheIoTandthecircuitdesign. Basedonhistoricaldatafrompreviousridesofsuchmaglevtrains,theauthorsextracted suspensionairgapcontrollaws,andasaresulttheydevelopedadaptivefuzzyrulesof maglevtrains’suspensionsystems.Alloftheseoperationsresultedintheverificationof themethod’seffectivenessbythetestingoffullscalemaglevtrains.Theauthorsstated thatforfutureresultstheyintendtofocusonthetuningrulesoftheproposedcontroller inordertoextendittoothersystems. Chenetal.[252]supportedmaintenanceactivitieswithintelligentconditionmoni toringsystemsthatensureavailability,reliability,andsafetyinrailwayoperations.The authors’ proposed method uses a combined technique of quantitative and qualitative characters,knownasacoactiveneurofuzzyinferencesystem,forthedetectionandiden tificationofthe10mostcommonfailuresofrailwaytrackcircuits.Theyappliedasingle

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audiofrequencyjointlesstrackcircuitfortheverificationofthesimulationmodel,andfor collectingfaultdata. SandidzadehandDehghani[382]employedaneurofuzzynetwork(coupledfuzzy systemsandneuralnetworks)forfaultdetectionandidentificationinatypicalaudiofre quencytrackcircuit,whichisahybridofknowledgebasedanddatabasedsystems.This kindofresearchisrelatedtointelligentconditionmonitoring.Theauthors’solutionwas generatedforinteractionwithlowprecisiondata,anditischaracterizedbythelearning abilityofaneuralnetwork.Themethodwasverifiedbyanalysesofmodeledandsimu latedregulartrackcircuits.Datawithandwithoutfaultswereusedfortrainingthealgo rithmoftheneurofuzzynetwork. InAnyakwoetal.[83],theauthorsdevelopedatwodimensionalwheel–railcontact model,withtheexclusionofsomevariablesofthecontactmodel’sgeometryinorderto reducecomputationaltime.Alookuptablewasappliedtoordinarydifferentialequations representingthedynamicbogiebehavior,andasaresultcomputationtimewasshorter. Thismethodcanbeappliedforfastandrealtimesimulationsofhybridintelligentsys tems(hybrid,accordingtotheauthors,consistsofmathematicalmodelingandcomputer simulationtechniques).Anovel2Dwheel–railcontactmodelwasappliedinthismethod. InEtchelletal.[383],theauthorsdevelopedaremotesystemforconditionmonitor ing,prediction,andpreventionincaseoffailuresofundergroundjointlesstrackcircuits. Suchasolutionallowedtheanalysisofalargescale,geographicallydiverse,intelligent systemthatcanconductrealtimeconditionanalysisofvoltageandfrequencyforcircuits. Márquezetal.[214]coupledintelligentconditionmonitoringwithreliabilitycen teredmaintenance,whichallowsthecollectionofbothdigitalandanalogsignalsobtained fromtransducersconnectedtoeitherpointtopointordigitalbuscommunicationlinks, dependingonsignalform.Theirresearchconductedfailureanalysis,dataacquisition,and incipientfaultdetectiononahydraulicrailwaylevelcrossingbarriersystem.Foreach barrier,notonlythehydrauliccharacteristics,butalsothemotor’scurrentandvoltage, hydraulic pressure, and barrier’s/rotary’s position were acquired. Data without errors wereobtainedwiththeuseofasinglecablefieldbusthatenablesallsensors’connection throughappropriatecommunicationnodestoabuscharacterizedbyhighspeed.There searchersusedARIMA(autoregressiveintegratedmovingaverage)modelsforthedetec tionoffaults.ThemodelsweretestedaccordinglywithJarque–BeraandLjung–Boxsta tisticaltests. ABayesiannetworkrelatedtotheintelligentconditionmonitoringofrailwaycate narysystemswasdevelopedinWangetal.[122].Thisnetwork’sparameterswerelearn ingbasedonhistoricaldata.Theauthorssuggestedthefollowingtypesofmeasurements forconsideringthementionedmonitoringsystems:wirestagger,contactwireheight,pan tographheaddisplacement,pantographheadverticalacceleration,andpantograph–cate narycontactforce.Theauthorswere curiousabout boththephysical meanings ofthe mentionedparametersandthecorrelationsbetweenthem.Theresearchresultedincate naryconditionlevelidentification. AsJoshietal.[46]mentioned,anintelligentconditionmonitoringsystemrequiresa properdatastoragesystemforthedetectionofincipientfailuresreportedduringthecon ditionassessmentofdistributedgenerationsystems.Suchsystemsarealsousedinrail transport.Theauthorscurrentlysurveyedusedapplicationsandvarioussoftcomputing methodsforconditionmonitoringinthecontextofintelligentsystemsusedfordiagnoses ofdistributedgenerationsystems. Amongthelesstypical—yetstillimportant—researchproblemsfromthecities’point ofview,thecleaningoflightrailtransportareascanbeconsidered.InDongetal.[194], theauthorsdevelopedaroad–railprecisioncleaningvehicledesignedtocleanthegroove rails of modern tramways. The authors developed an overall mechanicalstructure for suchavehicle,aswellasanelectricalcontrolsystemwithahighdegreeofautomation and,moreover,theyproposedaconditionmonitoringsystemandhuman–machineinter face system. Combining both a programmable logic controller and a touchscreen

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industrialpersonalcomputerallowedthemtorealizeanautomaticintelligentsystemfor cleaningbasedonimagerecognitionthatcoupledwithautomation,improvedtheeffi ciencyofcleaningtherailandreduced—forexample—waterconsumption,whichisused forcleaning.Theauthorsplannedtocontinueusingimagerecognitiontechnologyinor dertoimprovetheclassificationofgarbagebydeeplearningwithimagerecognition. In Sun et al. [384], the authors developed an intelligent conditionmonitoring methodforrailwaypointmachines(equipmentthatenablestheswitchingofroutesfor trains)basedonsound/noiseanalysis.Theauthorsusedthefollowingmethodsandtech niques:particleswarmoptimization,supportvectormachines,knearestneighbors,ran domforests,andnaiveBayesclassifiers;supportvectormachineswerestatedtobethe bestofthisgroupforidentificationaccuracyandcomputingcost. AsKarakoseandYaman[174]mentionedafterAazametal.[385],Industry4.0tech nologyprovidedthedevelopmentoftheInternetofThings(IoT),bigdataanalytics,smart machines,cloudbasedmanufacturing(CBM),andcyberphysicalsystems(CPSs).Theau thorsdevelopedacomplexthermographymethodcoupledwithfuzzymethodsforthe railwayandpantographcatenarysystem,whichresultedinanapproachforpredictive maintenanceonelectricrailwaysthatreliesonacomplexfuzzysystembasedthermogra phy.Acoupledapproachusingthermalimageandsignalprocessingmethodswasap pliedbytheauthors.Thereasonfortheproposalofthefuzzyapproachwasthatitwas suitableasamaintenanceestimatorwhenseasonalweatherconditions,externalenviron mentalconditions,daylight,andespeciallyintermittenteffectssuchastrainspeedwere takenintoconsideration.Moreover,theauthorsmentionedthatthermalchangesimpact bothrailsurfaceandpantographcatenary;therefore,theysuggestedthecouplingoffuzzy andthermalimageprocessingmethods.Moreover,inKnightPercivaletal.[372],theau thorsobservedthattheinterdisciplinaryfieldofsystemsengineering—inparticulartrans portationsystems—isextendedandsupportedbyIndustry4.0.Industry4.0,accordingto theauthors,focusesonthedesignandmanagementofcomplexsystemsduringtheirlife cycles,withconsiderationoffourdesignprinciples—namely,interoperability(communi cationandconnectionbetweensystems,devices,andsensors),informationtransparency (integrationofsensordatawithadigitaltwinofaphysicalsystem),technicalassistance (visualizationofcomplexdataandreductionofworkload),anddecentralizeddecisions (autonomyofcomplexsystemsthatcanadapttothespecificenvironment).InKnight Percivaletal.[372],asaconsequenceoftheimplementationofsuchprinciples,theauthors developedacustombuilt,lowcostmagneticfielddetectionsystemfortrackcircuitcon ditionmonitoring,mountedunderthebodyofarailvehicle.Theauthorsusedelectro magneticinductionasasensingtechnique,andtheconsequentlyobtainedmeasurements werenonintrusiveandgalvanicallyisolatedfromthetrackcircuit.Theauthorsdeveloped thephysicalsystemandaccompanieditwithavirtualdigitaltwinoftherailnetwork integratingthesensordata. Solutionsusedinrailwayconditionmonitoringarealsotransferabletootherfacili ties.Forexample,theauthorsofKoenigetal.[16,386]mentionedthatvibrationmonitor ingwasappliedtomeasuremovementinbothstaticstructuresandinrotatingequipment suchaswheels,andtheseauthorsdevelopedthisapproachtobeappliedtothestructure oftrackscarryinghighspeedrotatingequipment—namelybaggagecartwheelsatanair port.Theideawasusedbecauseunpredictablebreakdownsareaseriouschallengeatair ports,especiallywhenwheelsbecomeblocked,andasaconsequence,massivederailment ofcartsoccurswithinconnectiontunnels.Conditionmonitoringtopreventbaggagecart failureinrealtimewasappliedbytheauthors,andwasadditionallyenrichedbyautono moussystemslinkedtoartificialintelligenceandIndustry4.0airportlogic.InMetsoand Thenent[387],theauthorsalsoinvestigatedaspectsofconditionmonitoringinconnection tomaintenance4.0forairtransport;therefore,itmaybesuspectedthatthiswouldbea newtrendintheconditionmonitoringofrailtransportfacilities. InXuetal.[388],theauthorsdevelopedaninsulationmonitoringmethodforrailway signalingpowersystems(signalingpowerdistributionnetworkswithmultiplebranches),

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statingthatintelligentconditionmonitoringandaccuratefaultlocationaresignificantfor railwaytransport.Theauthors’methodallowsforthecontinuousmonitoringoftheresis tiveearthleakagecurrent,andindicatesthelocationsofleakagepointsbytrackingthe rectifieddifferencepattern.Theauthors’methoddidnotrequireanexternalpowersup plyorsignalinjection. The term Industry4.0 is interconnected with robotics; therefore, rail transport re searchersalsoshareaninterestinthisaspectofengineering. Ahmadianetal.[389]investigatedconceptstodecreasethedependenceonbonded insulatedjointsforrailwaytrackmonitoring.Theauthorsexaminedtheuseofinsulated jointsanddiscussedtheutilizationoftrackcircuitsaccordingtoknowledgeonacoustic methods,reflectometry,androbotics.Theauthorsconsideredautonomousvehiclesen richedwithGPStobeaformofrobotics;therefore,thisisonesuchearlypublishedpaper consideredwithinthistopic. InRowshandeletal.[246],theauthorsmentionedvariousdevicesthatareusedin the rail industry for condition monitoring—namely, inspection trains, dualpurpose road/trackvehicles,andhandheldtrolleysutilizingultrasonic,eddycurrent,andalternat ingcurrentfieldmeasurements,ormagneticfluxleakagesensors.Theauthorsalsomen tionedthatmostdevicesandsystemsprovidedataondefectlocations;however,thein formationondefectsizesisrarelyindicated.Therefore,theypresentedasystembasedon robotics,whosepurposewasthedetectionandcharacterizationofrollingcontactfatigue cracksinrailtracks.Theauthors’systemconsistedofamechanizedcart,aroboticarm,a lowcostdistancelasersensor,andacommerciallyavailablecurrentfieldmeasurement system.Whenadefectwasdetected,aroboticarmlocatedonatrolleywouldperforma detailedtwodimensionalscanofthecharacterizationdefects.Thiswasquiteanewsolu tion, and especially important at the time when the Industry 4.0 concept began to be broadlyanalyzed.

Table4.Thelistofreviewpapers’specifickeywords.

Occur TotalLink Keyword AppearanceofaParticularKeywordintheFollowingPublications rences Strength Varioustypesofsensors 3Daccelerationsensor 1 0 2014:[198] accelerationsensors 1 0 2018:[268] accelerometer 4 3 2020:[263],2014:[275],2012:[270],2011:[271] 2020:[142,263,288],2019:[50,281,284,289],2018:[110,278,279,282],2017: accelerometers 24 13 [146,280,390],2016:[166,276],2014:[91,242],2013:[274],2010:[118,119], 2007:[153,154],2006:[156],2001:[269] acousticemissionsen 4 1 2017:[165],2016:[166],2014:[168],1995:[286] sors angularratesensor 1 0 2014:[259] contrastsensor 1 0 2011:[291] eddycurrentsensor 1 0 2019:[233] electricsensingdevices 5 6 2018:[297],2017:[294],2014:[293,296],2012:[37] 2020:[190,308],2019:[304,306],2017:[294,295],2016:[298],2015:[303], fiberopticsensors 12 9 2012:[37,301],2006:[299],2003:[353] fiberoptics 6 4 2019:[306,358],2017:[295],2016:[359],2006:[103,299] fiberBragggratingsen 5 3 2016:[302],2012:[300,301],2008:[160,161] sors 2020:[263],2019:[50,289],2014:[91,242],2013:[274],2008:[205,206], gyroscopes 11 7 2007:[153,154],2006:[156] heterogeneoussensors 1 0 2016:[321]

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2020:[322],2019)[50,374],2017:[350],2012:[116],2011:[19,117,245], inertialsensor 9 2 2009:[45] inertialmeasurement 1 0 2019:[289] sensors infraredproximitysen 1 0 2013:[292] sor laserscanning 4 0 2016:[93,94],2014:[324,327] laserdisplacementsen 1 0 2019:[326] sors lasersensor 1 0 2011:[245] MEMSsensors 1 0 2019:[284],2017:[280] multiplesensors 1 0 2018:[328] onboardsensors 1 0 2020:[349] opticalfibers 6 8 2019:[304,305,358],2017:[294],2016:[298,302] opticalsensors 3 3 2018:[360],2016:[354],2007:[153] opticalfiberBragggrat 1 0 2012:[300] ingstrainsensor optoelectronicandpho 1 0 2018:[360] tonicsensors piezoelectricsensors 1 0 2018:[297] pressureandtempera 1 0 2017:[329] turesensors rechargeablesensors 1 0 2019:[143] remotesensors 2 0 2017:[49] sensor 7 2 2020:[307,338–340,363,371],2019:[50,287] 2020:[371],2019:[17,47,50],2018:[357],2014:[334],2013: [272,273,292,333],2012:[6,132,148,203,332],2011: sensors 26 17 [19,38,105,117,245,246,291],2010:[330],2009:[106,107],2008:[108,109], 2007:[26,53,153],2006:[159,299],2002:[52,331],1995:[286] sensordatafusion 3 2 2018:[323,337],2017:[335] sensorfusion 4 4 2020:[263],2018:[323,336,337] sensornetworks 5 7 2018:[267],2017:[49,293],2014:[294],2011:[245] 2020:[142,363],2019:[17,47,143],2018:[144–146,361],2016: sensornodes 13 26 [344,354,355],2014:[293] straingaugesensor 1 0 2012:[100] strainsensors 3 3 2016:[315],2012:[300],2012:[100] vibrationestimating 1 0 2018:[283] sensors wiredsensor 1 0 2018:[23] vibrationsensor 2 4 2020:[340],2018:[323] vibrationsensors 4 7 2020:[340],2019:[17],2018:[264],2013:[272,273] Wirelessandonlinecommunication Bluetoothcommunica 2 0 2012:[341],2011:[342] tion 2020:[322,351],2019:[347],2018:[112,346],2017:[350],2016: GPS 19 3 [89,90,344,345,352],2014:[91,92,242],2013:[273],2011:[204],2008: [205,206,343] mapmatchingalgo 4 3 2014:[91,92],2011:[204],2008:[205], rithm smartphones 4 0 2020:[349,351],2017:[350],2015:[348]

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wirelesscommunica 2 7 2020:[362],2018:[145],2015:[32] tions wirelessnetworks 2 4 2016:[354],2010:[379] wirelesssensornetwork 3 9 2019:[17,47,143] wirelesssensornetwork 2018:[23] 2 2 (WSN) 2018:[283] wirelesssensornet 1 5 2015:[32] works(WSNs) wirelesssensornet 2019:[17,47,143],2018:[23,144,145,283,357,361,380],2016:[354,355], 15 28 works 2015:[32],2014:[356] wirelesssensornode 4 8 2020:[363],2019:[143],2018:[144,145] Currentaspectsofindustry4.0 2020:[5,369,371],2019:[368],2018:[65,112,175]2015:[127,367],2013: artificialintelligence 12 7 [365],2012:[364],2006:[290] bigdata 10 8 2020:[378],2019:[9,255,373–376]],2018:[256–258],2015:[126] Industry4.0 5 0 2020:[174,372,387],2019:[16,368] intelligentcondition 5 0 2020:[381],2018:[122,388],2015:[214],2013:[382],2006:[252] monitoring intelligentsystems 5 1 2020:[46],2019:[194],2014:[129,383],2013:[83] InternetofThings 7 12 2020:[378,381],2019:[374],2018:[65,145,268,380] InternetofThings(IoT) 3 6 2020:[378,381],2018:[65] robotics 3 1 2016:[321],2011:[246],2005:[389]

5.Conclusions Asmentionedinthispaper’sintroduction,theapproachesappliedtothecondition monitoringofrailsystemshavemuchevolvedinrecentdecades.Observationofthese changesovertheanalyzedperiodoftimehasledustodefinethisreviewpaper’saim.We formulatedtworesearchquestions,namely: RQ1:Whatarethecurrenttrendsinconditionmonitoringapproaches,andhowhave thesetrendsevolvedoverthelastdecades? RQ2:Whatarethefutureresearchdirectionsandperspectivesintheconditionmon itoringofrailtransportsystems? Allofthepresentedresults,discussedbothintheformofbibliometricperformance analysisandsystematicliteraturereview,haveheadedtowardfindingtheanswerstothe twomentionedresearchquestions.Already,intermsofhighlightingthesubsectionsin thispaper,theauthorshaveidentifiedsomethematicgroupsinwhichresearchersare interested,inthecontextofwidelyunderstoodconditionmonitoringapplicationsinrail transport.Thesethematicgroupsofissueswereasfollows: varioustypesofsensorsusedindifferentsolutionsappliedtoconditionmonitoring inrailwaytransport, wirelessandonlinecommunication,andtherebycurrentaspectsofIndustry4.0,ap pliedtoconditionmonitoringinrailwaytransport. Thelattergroupofissueswouldseemespeciallysurprisingatfirstglance,because transportissuesareseeminglynotdirectlyrelatedtothetopicofIndustry4.0asanidea aimingatthefullautomationoftechnologicalandmanufacturingprocesses.Eachoneof thesethematicgroupsofissueswasdiscussedinachronologicalmanner;therefore,Sec tion4ofthisreviewpaper—comprisingsubsections4.1–4.3—answersthequestionabout currenttrendsinconditionmonitoringapproachesandtheirevolutionoverthepastfew decades.Inthesearchforcurrenttrends,thekeywordanalysisthataccompaniedthede scriptionsinSection4wasanundeniablemeansoffindinganswers,andthisculminates inTable3andFigure13;inTable3,quantitativeanalysisofresearchpapersrelatedto

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specifickeywordsisindicated,whileinFigure13,quantitativeandqualitativeillustration oftherelationshipsbetweenspecifickeywordsispresented.Itisalsoimportant,forex ample,toanalyzethecountrieswithinwhichtheresearchwascarriedout,includingin particularthecradleofrailtransport—theUnitedKingdom—andthecountriesinwhich highspeedrailwaysystemsaredevelopingonalargescale,asshowninthebibliometric analysisandsummarizedinFigure12andTable2.Alloftheinformationpresentedin thisreviewpaperfocusedonthespecificevolutionofsolutionsconnectedtocondition monitoring.Atpresent,maintenancehasbeengettinggraduallymoresupportfrommeth odsconnectedtosensorapplication,andtheincreaseduseofsensorshasledtothecur rentlydiscussedtechniquesfocusedonIndustry4.0.Anenormousincreaseinamountsof generated data—known as big data—requires diagnostic procedures to be automated. Suchachallengeinducedtheneedfornewandimprovedmethodsofautonomousinter pretationofconditionmonitoringdata(mainlyvibrationmethods). Thesecondresearchquestionstatedbytheauthorsofthispaperwas“whatarethe futureresearchdirectionsandperspectivesintheconditionmonitoringofrailtransport systems?”;thesedirectionsandperspectivesmightbeexpressedbytheindicationofpar ticularresearchagendas. Inertialsensorssuchasaccelerometersandgyroscopeshavebeenappliedtothecon ditionmonitoringoftransportationresourcessincetheearly1990s,Chiaetal.[50].There fore,itcanbestatedthatthisisamuchdiscussedtopic,anditisworthnotingnovelfields ofexploration.Theapplicationofnewconceptstriggersanumberofpotentialresearch agendasthatmaybedevelopedinthecomingyears,inadditiontothosethatarestillin progress.Oneoftheserelativelynovelresearchagendasthatareconnectedtochangesin conditionmonitoringresultsfromthedevelopmentoftheconceptofIndustry4.0.This researchagendacanbeexpressedasarequirementfortheautomationofdiagnosticpro ceduresandmethodsofgeneratinglargeamountsofdata,whichdrivestheneedformore sophisticatedmethodsofautonomousinterpretationofvibrationreliantconditionmoni toringdata.Inturn,alargeamountofmonitoringleadstothecollaborativetermofIn dustry4.0asbigdata.Thisisworthconsideringinaccordancewiththepastdecade’s discussionofparticularaspectsofbigdataappliedinconditionmonitoring,suchasvol ume,velocity,variety,veracity,andvalue.Furthermore,discontinuanceofonboarddata computingisstillachallenge,andseemstobearesearchagendaforthenextseveralyears, especially given that the quantity of data continuously increases as an exponential or powerfunctioninrelationto—forexample—thenumberofsensorsusedandtheirmeas urementdirections.Thismaybesupportedbytheapplicationofvariousmethodsofarti ficialintelligence.AnotherinterestingresearchagendahighlyconnectedtoIndustry4.0is theInternetofThings,ascertainabovementionedstudiessuggest. Moreover,fromtheviewpointofresearchers,itissignificanttoreducethenumber ofsensorsasfactorsgeneratingadditionaldatainhugeamounts.Someauthorsgoeven further,wonderinghowtoanalyzetheconditionoftracksandvehiclesotherthanbyadd ingmoresensorstorailroadsystems(e.g.,usingpassengers’phonesforthispurposeis interesting).Aseparateissueistheanalysisofdifferentmeansofcommunicationbetween differentelementsofmonitoringsystems,whichisdiscussedinthesecondsubsectionof Section4. Ingeneral,aseparateresearchagendaistoconsidertheappropriatenumberofsen sorsusedforaspecificneed,andtodeterminewheretolocatethesesensors. Attheendofthispaper,itisalsoworthmentioningsomeofitslimitations.Oneof theseisthemannerinwhichtheliteraturereferencesaresegregated.Athematicconven tionwasadoptedinthisregard,inlinewiththekeywords,althoughwithineachtopic (themes) the successive works were described in chronological order. Because of this chronologicalorder,thismayinsomecasescreateasenseofapparentdifferentiationof themes,especiallyincaseswherekeywordsareconsolidated(whichnecessarilyoccursin thecaseofmostofthecitedpublications,asseveralkeywordsareassignedtoeachpubli cation).Inaddition,infutureanalyses,itwillbeworthcommentingontheincreasing

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numberofsensors,theirdiversity,andtheamountofdatathatwereacquiredusingthese sensors,whilealsoelaboratingonthegradualabandonmentoftheuseofsensorsintrack andrailvehicleconditionmonitoringstudies.Infutureconsiderations,itwouldbeworth separatingthetopicsoninfrastructureandmeansoftransport—focuseddirectlyonlight railvehicles,urbanandlongdistancerailtransport,andhighspeedtrains—andaddition allyincludeabreakdownbycountry. Moreover,itisworthnotingherethattopicscenteredontheconceptofIndustry4.0 havejustrecentlyenteredthefieldofvehicleandtrackconditionmonitoringinthecase ofrailtransport.However,“[t]hebiggestchallengeforconditionmonitoringisensuring selectionoftherighttechnologies”,BrantandLiang[7].

AuthorContributions:Conceptualization,M.K.;methodology,M.K.;writing—originaldraftprep aration,M.K.andR.M.;writing—reviewandediting,M.K.andR.M.;visualization,M.K.;supervi sion,M.K.;projectadministration,M.K.;fundingacquisition,M.K.Allauthorshavereadandagreed tothepublishedversionofthemanuscript. Funding:Thisresearchreceivednoexternalfunding. InstitutionalReviewBoardStatement:Notapplicable. InformedConsentStatement:Notapplicable. DataAvailabilityStatement:Notapplicable. Acknowledgments:Tothosewhomatter. ConflictsofInterest:Theauthorsdeclarenoconflictofinterest.

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