ADECISIONSUPPORTSYSTEMFORRAKELINKING ANDRELATEDOPERATIONALDECISIONS+ ∗ JaiMruga,AmitArorabandNarayanRangarajc.∗ a,bDepartmentofMechanicalEngineering cIndustrialEngineeringandOperationsResearch IIT-Bombay,Mumbai400076 1 INTRODUCTION Rakelinkingisthetermusedforthedecisionofassigningphysicalrakestocertainsets ofscheduledservicesonaregularbasis.Theparticularrakelinkingpatternsthatare chosendependonanumberoffactors.Inpassengeroperations,thetime -tableof scheduledservicesisthetarget,whichhastobemet.Certainconstraintssuchas availabilityofmaintenancefacilities,safetyconsiderationsandnormsofoperationcanbe consideredtobeprimaryones,whilemaximisingreliabilityofservice sandincreasing operationalflexibilitycanbeconsideredassecondaryconsiderations.Subjecttothese, efficientrakeutilisationisanobjective,whichcanbereasonablytackled.Thisissueis presentedinitsrealisticoperationalcomplexity,withas uggestedmethodologyfor evaluatingcertainrakelinkingoptions. Theissueofrakelinkingisseeninitsinter -relationshipwiththethreecritical operatingdatabasesoftheIndianRailways:the timetable,whichrepresentsthedemand side,therakelink,whichcapturesthesupplyallocationoftherollingstockbylistingthe rakelinksandthe platformandmaintenancelineoccupancycharts whichprovide informationontheallocationofterminalfacilities.Inviewofthecomplexnatureofthe decisionsinvolved,aneedwasfelttoprovidesomekindofmanagementsupport.This wouldallowthedecisionmakertounderstandthetotalityofthesituationinmore concreteterms.Anattempthasbeenmadeinthisdirectionbyimplementingaprototype DecisionS upportSystem(DSS)termedTRIMS(TerminalRakeInformationand MaintenanceScheduling).TRIMScapturestherelevantinformationbyintegratingthe timetable,rakelinkbook,platformandmaintenancelineoccupancycharts. Theobjectiveistoultimatelyp rovideauserwithacomprehensiveanalysisof coachingstockallocation,overtheentiresystem.TRIMSconsistoftwomodules - RakeInfoand Scheduler.Byintegratingtheoperatingdatabases, RakeInfoprovides comprehensiveinformationaboutservicesand rakecycles. Schedulerallowstheuserto scheduleallthestipulatedservicesontheavailableplatformandmaintenancelines. Further,theschedulethusarrivedisrepresentedinavisualformat(GanttChart).TRIMS hasbeenimplementedinCinaUNIXenvironmentonX-Windows. +ThisworkwasdonewhenthefirsttwoauthorswereatIITBombay.TheauthorsthankMangeshS. Gharote,RavindraGokhaleandHrishikeshKarmarkarforhelpinpreparingthisdocument,andProf G.Raghuram(IIMAhmedabad)andseveralrailwayofficialsfordiscussionsupport. ∗Correspondingauthor.E-mailaddress:[email protected] 1 WefocusonlongdistanceMailExpresstrainsandtheirlinkingpatternsinthis study.AnearlierstudybyK.V.RamaniandG.Raghuramisthefoundationforthis analysis.Theirreportillustratestheideasthatarediscussedwi thexamplesfromCentral andWesternRailwayoperationsinMumbai.WenotethatthetimetableonIndian Railwaysisanevolvingdocumentandmanytrainnumbersandtimingsreferredtohere, arenolongervalid.Theprinciplesdiscussedherecontinuetobevalid. 2 MAJORCONSIDERATIONS Webrieflydiscussthemajorconsiderationsinthepreparationofarakelinkaspartofthe timetablingexercise,seenasawhole. 2.1 TIMETABLE: Thetimetableprovidesascheduleofregularservicesandthetypeofse rvice(rake composition)providedineachcase.Ifwetreatthisasgiven,thefirstconsiderationin rakelinkingisthataservicecanbelinkedtoanotherserviceatacertainterminal, providedasuitabletimegapexistsbetweenthearrivalofoneand thedepartureofthe other.Sometimes,departuretimesofservicesarenotfixed apriori andareactually synchronisedwiththatofthelinkingtrain.Thisisespeciallytrueinrakelinksinvolving shortinter-cityruns. 2.2 MAINTENANCENORMS: Athumbrule,whichemergesfromananalysisofdifferentrakelinksindifferentzonal railways,isthat,forMail/Expressoperations,afteranupperlimitof2500kmsarake mustgetaminimumtimeof6hrsforPrimaryMaintenance(PM).Similarlyaftera distanceof1500kms,itmustgetaminimumof1to1.5hrs.ofSecondaryMaintenance (SM).Fortrainsrunningbeyond2500kms.,moreprimarymaintenancetimewouldbe required.Itisgenerallyobservedthatalongdistancetrainidlesforaminimumof5hours beforeitbeginsanotherservice.Duringthistime,itcouldundergoprimaryorsecondary maintenance,dependingpartlyonthemaintenancefacilitiesavailableatthatterminal. Ideally,maintenanceactivityshouldbeviewedinthecontextofarakecycler atherthan associatingitwitheachindividualservice.ReferAppendixAforasummaryofthese empiricalobservations. GivenanaveragespeedofMailExpresstrains,asimplerelationshipcanbestated whichlinksthemaintenancenormsdependingondistan ce,therunningtimesand maintenancetimes.Thisresultsfromthefactthatanaverageofabout25%ofthetotal rakecycletimeisavailableformaintenance.Aconvenientratioisthatmaintenancetime is1/3ofthetotalruntime. Thesefigureshavebeenarrivedat,afteranalysisofdifferentrakecycles.Acloser lookattherakecycleswouldrevealthatthereissubstantialvariationintermsofthe frequencyofmaintenance.(Example:IncaseofTrivandrum -GuwahatiExpressPMis providedafterarun of3582kms.and65hrs,ontheotherhandintheMumbai –Pune- Mumbaisector,itisapproximately400kms.and8hrs.) 2 2.3 MAINTENANCEFACILITIES: Especiallyatcongestedmetropolitanterminals,availabilityofspecificmaintenance facilitiesisamajorconsiderationinrakelinking. 1.PrimaryMaintenance(PM) , isdoneatthebasestationtowhichatrainisallotted. Thisrequiresapitlineforthescheduledtime.Primarymaintenanceinvolvestechnical servicessuchascheckingoftheaxleboxesand repairingoroilingthemifnecessary. Besidesthesetherearecertainchecks,whicharecommonwithsecondarymaintenance likecheckingofthetrolleydefectsandspringcracks.Brakesarealsocheckedtoensure thatthebrakeefficiencyiscloseto100% .Itisconsidereddesirablefromsafety considerationsthatprimarymaintenanceisdoneduringdaytimehours,wherever possible. 2.SecondaryMaintenance(SM) ,isdoneatthemajorterminalwherearakevisits.This mayrequireanemptysidingforthesc heduledtime.Otherthanchecksonthebrake springsetc.,italsoinvolvesserviceslikecleaningandwashingofrakes. 3.ReadytoRunInspection(RRI), isdoneatmajorstationsalongtherunwhich sometimesrequiresplatformavailabilityforthesched uledduration.ForPMandSM, shuntingtimehastobeprovidedfortheraketobehauledtotheyard.Primary maintenancefacilitiesarelimitedandrakelinkingissometimesdonespecificallywith theschedulingofsuchfacilities(referClass(C)-(ii),(iii)and(iv)inSection1.5below). 2.4RAKECOMPOSITION RakeCompositionreferstothenumberandthetypeofcoaches(AC,SecondClassetc.) inarake.Trainservicesusevaryingnumberofcoaches.Further,twoserviceshavingthe samenumberofcoachesm ayhavedifferentrakecompositiondependingonthe combinationofcoachtypeused.Oneofthemajorchallengesinachievingimproved utilisationofcoachesistheabilitytostandardisearakeintermsofitscomposition.To linkupsomeparticulararrivalserviceataterminaltoadeparture,itisdesirablethatboth theseservicesshouldhaveidenticalrakes.Intheabsenceofidenticalrakes,linkingwill stillbepossiblebutsomeshuntingwillberequiredi.e.itmayberequiredtoattachor detachs omecoachesfromaparticularrake.Thus,inmanycoachessavingmaynotbe possibleintermsofrakesbutintermsofcoaches.However,standardisationofrakes(i.e. alltheservicesonaparticularnetworkhavethesamerakecomposition)facilitatesrak e linking. 3 OTHERCONSIDERATIONS Twootherrelevantconsiderationsarediscussedhere,ofaqualitativenature,the reliabilityofservicesthatareplannedinatimetable,andtheoperationalflexibilitythata rakelinkingdecisionprovides. 3.1RELIABILITYOFSERVICES 3 Rakelinksoftenhavetoacknowledgethefactthatuncertaintyofoperationsare sometimeschronic,andtillsuchtimeasthereliabilityofcertainservicesimproves,there mighthavetobespecialprovisionsformaintainingpunctual services.Thisresultsin certainservicesbeinglinkedtootherservices(usuallythesameserviceinthereverse direction)afteralmost24hours.Essentially,astandbyrakeisprovidedonsuchservices. ReferClass(C)-(ii)inSection4below. 3.2OPERATIONALFLEXIBILITY Aconsiderationsimilarto(2.1)aboveisthatoperationaldelaysareoftendirection specific(e.g.duetodisruptionsonaparticularline).Thissuggeststhatrakelinks,which connectservicesalongthesameline,arebetterfor operationalflexibility.Thereasoning isthatifservicesondifferentlinesarelinked,thentherearepossiblecascadingeffectsof delaysanddisruptionsondifferentlines.If,onthecontrary,servicesonthesamelineare linked,thennomorethan theunavoidabledelaysofservicesonthatlinealoneare incurred.SeeClassesC -(iii)andC -(iv)inSection4belowforexamplesofthesetwo typesoflinks. OtheroperationalstrategiesforbetterreliabilityarediscussedinAppendixB 4 RAKEUTILISATION Wenowcometoamajorconcernofrakelinking,fromasupplyperspective.Thisisthe objectiveofmeetingtheserviceobligationswithasfewrakesaspossible.Thisisclearly desirablefromthepointofviewofassetutilisation.Ashasbeen clearlypointedoutby RamaniandRaghuram,themostdesirablestrategyfromthisperspectiveistolinkeach serviceateachterminalwiththefirstavailableservicethatmeetsmaintenanceandother majorconstraints.ThismaybereferredtoastheFCFS policy.(TheanalysisbyRamani andRaghuramgoesfurther,suggestinglinkingevenwhenrakecompositionsarenot exactlythesame,incaseswherethecostsofsomeshuntingoperationsmaybefar outweighedbythegainsintermsoffewercoachesused,systemwide.) Letusnowsupposethatrakesareallottedtoservices(insomelinkedmanner,basedon theFCFSlogicoranyotherscheme).Wecanthinkofanumberofwaystoproceed beyondthisstage: i) Theintroductionofnewserviceswiththesamerakesuppl ycanbe considered.Thishasindeedleadtoanumberofintercityservices,which makeeffectiveuseofthelayoveroflongerdistancetrains.(Seeexamplein Section4:(B)-(iii)(a)foranexampleofthis.) ii) Thedirectionalconsiderationin2.2above,orotherconsiderationscanbeused toexploitthefactthatthereare,infactanumberofschemes(otherthanthe