Developments on the Great Western Railway.” by ALFRED THOMAS BLACHALL,M

Home , Block post, Double junction, Railway signalling

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28 February, 1911. ALEXANDERSIEMENS, President, in the Chair.

(Paper No. 3931.)

“ Railway-Signalling : Developments on the Great Western Railway.” By ALFRED THOMAS BLACHALL,M. Inst. C.E. THE Paper on railway-signals by the late Mr. R. C. Rapier, read at the Institution in 1874,’ described very fully the evolution of railway signalling up to that date, and Mr. A. M. Thompson’s Paper on the signalling of the London and North Western Railway, in 1885,2 showed advances madein railway signalling and interlocking during a later period. In the present communication it is proposed to refer to some further developments which have taken place in recent years upon the Great Western Railway, that being the line with which the Authoris connected and best acquainted. Although signalling and interlocking had longbeen an established feature of theGreat Western Railway, there were stillin 1889 many stations and junctions where the signalling was, according to modern ideas, very insufficient, and at which there was little or no attempt to concentrate or interlock the levers. The Regulation of Railways Act passed in that year gave power to the Board of Trade to require railway-companies to set their houses in order in various ways, and amongst the Orders made by the Board of Trade in virtue of thesepowers was onedated the 26thNovember, 1890, which required the Great Western Railway Company to adopt the block system on allits lines, with certain exceptions, andto provide for the interlocking of points and signals, also subject to certain exceptions. Although it was not found possible to comply with these require- mentswithin the actual times specified in the Order, they were fulfilled with all reasonable speed,and within a short time thewhole of the railway had been properly and efficiently signalled. As the

Minutes of Proceedings Inst. C.E., vol. xxxviii, p. 142. * Zbid, vol. lxxxii, p. 166.

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 154 BLACKALL ON RAILWAY-SIGNALLING. winutes of GreatWestern Railway had been extendingand improving its interlocking and signalling arrangements for many years, the effect of the Order was simply to expedite somewhat the completion of that work of signalling its line throughout which the company had already taken in hand, A few years ago, the company, having in mind that the signalling of its railway comprehended something more than the installation of semaphores, the connecting of rail-switches, and the interlocking of levers actuating them, andalso having in mind theextensive and increasing use of electrical appliances in signalling, decided that the co-ordination of the outdoor signalling and block signalling was a necessity ; it therefore placed the supervision of bothsignalling and telegrapharrangements under one control. It maybe of interest to recall that in the discussion on Mr. Thompson’s Paper, Mr. Spagnoletti, then Telegraph Superintendent of the Great Western Railway,advocated this course, and events have proved the correctness of his views. The Signalling Department controls the construction and maintenance of all signalling appliances, both mechanical and electrical, including telegraphs and telephones. The modern developments of signalling on the line arelargely in the direction of the increasinguse of electrical apparatusand appliances in connection with its outdoor signalling arrangements, andthe Author proposes, inter alia, to describe briefly some of these. IXTERLOCKINGFRAMES. For many years an interlocking frame designed by the Author’s predecessor (hisfather) was used, in which the interlocking of levers was carried out in a manner peculiar to the Great Western Railway ; but this form of interlocking could not be applied to conditional or special locking, and was suitable only forthe ordinary locking or releasing of onelever by another. For conditional- interlocking combinations tappet-locking had to be resorted to, and this led the Author to decide to use tappet-locking generally. With the exception of some smallerframes in which the interlocking between the levers is effected by means of what is known as stud- locking-which is reallyamodification of theother method- the tappet form of locking now generally employed on most railways is used.

ARRANGEMENTOF SIGNALS. It is of the greatest importance to place the signals so that they shall be easily readand understood by engine-drivers,and in

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] BLACKALL ON RAILWAY-SIGNALLING. 155 Figs. 1-8, Plate 5, is shown the generalplan upon which the signalsare placed withthis object in view. It will be seen that the general idea is to place each signal next to and upon the left- hand side of the line to which it applies, and to vary the height of thearms in accordance withthe importance of the lines. For instance, in Figs. 1 and 2 it will be noticed that the home signal at the facing points in each case has the arm for the main line higher than Lhe arm whichapplies to the branch, and to emphasize the distinction the former is placed upon the main post of the signal, while the arm for the branchis bracketed out. In Fig. 3, where the speed on both of the diverging lines would be the same, neither homesignal is upon the mainpost of the signal, but both are bracketed out to indicate the equal importance of the arms. In Fig. 4, where there is no branch line, but merely a turn-out to an avoiding line or siding, the principle of keeping the main-line arm on the straight post higher than that for the diverging line is again acted upon, and asthe loop line is comparatively unimportant thearm for it is kept lower than it wouldbe for an ordinary branch line, while it is also smaller. The signalling for parallel lines is shown in Fig. 5, where each signal is placed next to and upon the left-hand side of the line to which it applies. A junction at which fast running is on the straight road is shown in Fig. 6, where the speed is low on the branch line which goes off to the right, with a lower speed still upon the branch line going off tothe left, the arms upon the junctionsignal being varied in height accordingly. The signals for a " scissors " crossing in a station through which thereare four linesare illustrated inFig. 7. Theelevation will show how the signals are arranged to make them distinctive. No. 1 is the signal for the through line A, No. 2 is the signal leading from the through line A to the platform-line, No. 3 is the signal leading from the platform-line to the through line A, and No. 4 the signal for the straight platform-line. A double junction between parallel lines is given in Fig. 8, which shows the arrangement of the junction signals. The arrangement is on the same lines as in the othercases mentioned. It happens frequently thatit is not possible to arrange the signals as here described, owing to want of space between the lines, but it is very desirable in preparing plans for new lines or stations, and for other rearrangements of lay-out, to arrange the spaces between the lines so as to allow of the signals beingfixed in their properplaces.

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 156 BLACKALL ON RAILWAY-SIGNALLING. [Minutes of Signals upon gantries spanning the lines are read very easily by drivers,and can beplaced actuallyover the lines to which they apply. The expense, however, of a signal-gantry is veryconsider- able, and is hardly to be justified unless its provision is absolutely unavoidable.The disposition of thepoints at very compliated stationsand junctions renders it impossible, inthe majority of cases, to group many signals upon one gantry so that they shall 011 be intheir right places, because it seldom happens that all the facingpoints and fouling points which the signals protect come immediately under the gantry. On this account very few gantries are provided upon the Great Western Railway. It was formerly the practice torepeat the home signals of a junction at the distant, that is to say, each home signal had its corresponding distant, but inview of the fact that atmany junctions there is fast running on one line only, and that a train diverging from the main route must reduce its speed considerably, it was felt to be bad practice to give the same indication upon approaching a junction to a driver who was required to reduce the speed of his trainto, say, 15 or 20 milesper houras to a driver whowas permittedto run through the junction at full speed. It is now customary, therefore, at junctions where there is fast running on one line only to provide but one distant signal, and this signal is capable of beinglowered only when the road is set for the fast- running line. The driver of a train which has to leave this line at the junction, and in consequence to reduce the speed of his train, passes the single distant at danger.This arrangement has the advantage of ensuring that the driver of the train which is required to slacken speed is checked at the distant signal, and it is also an economical arrangement. At other places than at junctions, where the distant signal covers a portion of the line over which, or at a station through which there is a permanent reduction of speed, the distant signal is fixed permanently at danger in order to afford a standing warning to drivers that speed must be reduced. On the Great Western Railway this arrangement is made only when the speed is required to be reduced to 15 miles per hour or less, but it is a question whether it should not take effect for higher speeds, in fact, for any speed which is less than normal speed.

ELECTRICALAPPLIANCES FOR DISTANTSIGNALS. Until the last fewyears, it was customaryto provide electric repeaters in connection with such distant signals only as could not ordinarily be seen by the signalman working them, but it is now

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. proceedings.] BLhCKALL ON RAILWAY-SIGNALLING. 157 the practice to provide all distant signals with electric repeaters, for the reason that although in clear weather a signalman may have a goodview of his distant signal, there are frequently times when, owing to the stateof the atmosphere, he cannot see it, although there may be nothing to prevent him from seeing his stop signals, which of course are nearer to him.Electric repeaters are also provided for stop signals when they cannot be seen from the signal-box from which theyare worked.Controlled orslotted signals have the controls electrically repeated to the signal-box. There are tunnels and other places where the distant signal must of necessity be a very long way from the signal-box, and where there are consequent difficulties in working it satisfactorily. It is usual now in such cases to provideelectrically-worked signals operated from a primary battery placed at the post, and controlled by a relay which is operated from the signal-box. It has been found that dry cells are satisfactory and economical for this purpose, and with an efficient motor at thesignal this plan works well. Although from the point of view of running trains the distant signal is the most important of all, since it not only repeats the position of the stop signals ahead, but also indicates whether the block section ahead is clear or not, it is, unfortunately, the most difficult to keep in proper working order on account of the long length of wire-sometimes considerably over 1,000 yards-by which it is workedfrom the signal-box. This sometimes resultsin the signal-armdrooping and giving a false clear signal,and cases of drivers overrunning stop signals have resulted. A device designed to prevent this has been tried on the Great Western Railway and has proved successful, It consists in interlinking the distant signal with the block telegraph system in such a manner as to make it impossible for the signalman to give “line clear” to the station in the rear, unless at the time he attempts to do so his distant signal is properly at danger. In the Author’sopinion this is auseful development, as it ensures that the distant signal must have gone properly back to danger after the passing of one train and before the acceptance of another.The arrangement can be verycheaply applied by making use of the electric repeater circuit.

SPECIAL SIUNALS.

In order to avoid multiplicity of arms at a point leading to a number of routes a route-indicating signal, as illustrated by Fig 9, Plate 5, is frequently used, With such a signal a rectangular opal screen is attachedto the post below the arm and shows awhite ground, which at night is stronglyilluminated by a lamp at the

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 158 BLACKALL ON RAILWAY-SIGNALLING. [Minutes of back. The whole area of the opal screen is uniformly and effectively lighted by means of a specially-constructed lens and reflector, with which the lamp is provided. The indications showing the route to be taken are cut out on slides of galvanized sheet steel like stencils. In the normal position, when the signal is at danger, the slides are out of sight in the slide-box just below the screen, and they are raised in front of the screen when the levers operating them are pulled over in the signal-box. The arm drops to the “off” position when any one of the indicating slides is raised, and when the slide is lowered the arm returns to the “on ” position. The indications, which are clearly visible for a sufficient distance, appear white on a black ground. The opal screens are so constructed as to beeasily removablefor the purpose of cleaning.The advantages to be derived undercertain conditions from the adoption of this form of signal are many, and tend in many respects to simplification. It will be seen that only one arm or light, as the case may be, need be looked for, instead of its being necessary to pick out a certainsignal from a complicatedgroup. Further,a clear andunmistakable indication is given tothe driver as to his destination. Another important point is the reduction in first cost and subsequent upkeep as compared with the older system ; the number of arms and lamps is greatly lessened, and a large savingis effected in the consumption of gas or oil ; the labour involved in trimming lamps and cleaning spectacles is also reduced toa veryappreciable extent. Route- indicating signals are used chiefly at terminal stations, or at points where the speed of trains is comparatively low.On signals for fast-running lines separate arms are preferable. With regard to the use of outer home signals, at many block posts, chiefly junctions, it has been found that the absolute-block system of working hampers traffic considerably, inasmuch as it necessitates a mile of line in advance of the home signal being unoccupied before line clear ” can be given to the block post in the rear. The consequence is that in many cases delays to traffic occur. In order to avoid in such cases the necessity for the application of the warning arrangement, i.e., the acceptance of trains under the “ section clear but station or junction blocked ” signal, an outer home signal, mile inthe rear of theordinary home signal, is frequentlyprovided. The provision of this signal allows “line clear ” to be given to the Mock post in the rear so long as there is 4 mile of unoccupied line within the outer home signal. In order that a signalman maybe made aware when a train is waiting at the outer home signal, the line is track-circuited for a distance of 100 yards to the rear of that signal, and an indicator is provided in the signal-box which shows

U train waiting at signal ” whenever the track is occupied, while

through the agency of the track-circuit the block telegraph indicator is held in the “ train on line ” position. This arrangement has been found greatly to facilitate the traffic working while retaining in its integrity the fundamental principleof block working. At pointsleading from and to sidingsindependent disks are used, though the originalpractice was to provide a ground-indicator working with the points ; but the experience which has been gained from many inquiries into the causes of derailments at siding points has led theGreat Western Railway to provideseparate groundsignals for all such points,the ground-signals working independently of the points, as shown in Fig. 11, Plate 5. Such signals are provided with detectors upon the points, which ensure that the “off” signal cannot be given until the points are in theirproper position. A goods-loop train-indicator is used for the purpose of warning a driver of the condition of a refuge-loop which he may be entering, that is to say, whether any, and, if so, how many trains are already inthe loop. TheAuthor has designed the indicatorshown in Fig. 10, Plate 5, which is to be fixed upon the post of the signal for entering the loop. The case upon the signal-post contains a circular “ window”in whichnumbers appear, corresponding with the number of trainsadmitted into the loop. Thedisk carrying the numbers is revolved by means of a lever in the signal-box. There are notches in the curved guide of the lever, corresponding with the number of trains which the loop will hold ; and the numbers upon the disk in the case correspond withthese notches. When the loop is empty the lever is in its normal position, and “ 0 ” appears in the window. A train having been sent into the loop, the signalman pulls the lever to the first notch, which causes the disk to revolve partially, when“ 1 ” appears in the window ; and so on, untilthe maximumnumber of trainsthe loop will hold is reached. A lamp inside the case illuminates the window by night.

LIGHTING.

With the introduction of a green light for the ‘‘ all right ” or “ off” position of the signal-arms, in place of the white light which was previouslyemployed, some difficulty was experienced in obtaining a suitable green glass ; but this difficulty was overcome by adoptingthe recommendation of the Committeeon Colour Vision presented to Parliament in June, 1892, and using, for both red and green lights, glasses selected to correspond with the Board of Trade standard for similar lights at sea ; these glasses have been found to give every satisfaction.

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 160 BLACKALL ON RAILWAY-SIGNALLING. minutes of The increase inthe number of ground-disk signals andother subsidiary signals brought about a multiplicity of red signal-lights, with a consequentrisk of signalsbeing mistaken by main-line drivers. -The difficulty has been met by dispensing with red lights in ell subsidiary signals, such as siding, backing, and groundsignals. This has had the result of making it very much simpler for drivers to pick up their running-signals, as it is only such signals which now show red lighk atnight. In connection with signal-lamps, considerable progress has been madesince rape-oil was abandoned in favour of petroleum.The signal-lampburner which replaced the old rape-oil burner hada $-inchwick, consuming 19!, gallons of oil in 12 months. The question of reducing the consumption of oil was under consideration by the Great WesternRailway for some considerable time, and afew years ago,in order to find an effective lamp, arrangements were made on a branch line for testing various lamps as they could be obtained fromtime to time. This test was arranged inthe following manner : Two bracket signal-posts werefixed, capable of carrying eight signal-lamps, anddistance posts were provided at each 100 yards up to a distance of 1,000 yards. On the nights arranged for the tests,representatives of the locomotive , traffic- and signal- departments recorded their opinion of the lights at each 100 yards, andafter repeated trials of the variouslamps, it wasdecided eventually to adopt a lamp with a %-inch burner (with reflector) consuming 7 gallons of oil per annum.The conversion of the whole of the signal-lamps throughout the line was at once taken in hand, the result being a large saving in the consumption of oil. Atthat time the " long-burning " lamphad not reached its present state of efficiency, but since then improvements have been made inthe construction of thiskind of lamp. Aftertrials at a few stations and on branch lines, which proved that long-burning lamps were effective, arrangements have been made for their intro- ductionon the varioussections of the line,with the result that further considerable savingsin the lighting-staff and inoil are being effected. The lamps require filling only once a week, and trimming twice a week,, Gas-lighting for signals has practically been abolished so far as the Great Western Railway is' concernedexcept at one or twolarge stations, andit is now usedonly forspecial signals fixed on verandahs of stations and in other situations where inconvenience might be caused by the use of oil. It has been proved conclusivelythat oil as an illuminant for signals is far preferable to gas, being both cheaper and more reliable.

WORKINGOF SIGNALS. Many experiments have been made by the Author with automatic signal-wire compensators of various designs, but he has not found any which undertrial have provedto be quite satisfactory andreliable. The practice for a long tiFe now on the Great WesternRailway has been to provide in thesignal-box means bywhich the signalman him- self can readily adjustthe wires of his signals; and as signal-wires of short length seldom need adjusting, it is chiefly the distant-signal wires which require to be let out or taken in from time to time. This plan has worked verywell, and it has the advantage thatit places upon the signalman the responsibility for seeing that his wires are properly adjusted, as in order to do this he must watch the working

Fig. 19.

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Scale: Half Full Sire

of his signals ; while with an automaticcompensator there is a danger that the signalman will take for granted the correct working of his signals. In order to prevent the.necessity for keeping the wire too slack in cases in which the signals are at a distance from the signal-box, the Author has found it advantageous to insert two springs in the wire, one at the end nearest the signal, and one at the signal-box. These springs are shown in Figs. 12, Plate 5, and by means of them it is possible to keep the signal-wires tighter than would be possible without them, thuseffecting a considerable reductionin thebreakage of wires through sudden strain. A method of making the joints insignal-wires which has recently been adopted on the Great Western Railway is depicted in Fig. 19. By means of the special sleeve shown this joint can be made very quickly, and it is less liable to breakage than a joint made in the [THE INST. C.E. VOL. CLXXXV.] M

ordinarymanner. Short lengths of wire rope are used at the angles in the signal-wires, instead of chain as formerly. Similar preventionof wear and tearhas arisenby the use of a buffer- spring in thedown-rod of signals. Fig. 12 illustrates this springalso. The long wheel-base which is a feature of modern rolling stock hasresulted in a considerableincrease inthe length of facing- point, lock-bars, which from their original length of about 16 feet have now reached a length of 50 feet. The Great Western Rai1wa.y has adopted as standard practice the provision of assisting-springs in the rods working facing-point lock-bars. There are two springs, one of which is in compression whenthe bar is down on one side, and another which is in compression when the bar is down on the other side. When the bar is raised neither spring is compressed. It will be seen, therefore, that as the bar drops it compresses one or other of the two springs, and, so to speak, stores up energy which assists the movement of the bar the next time it is required to be thrown over. This arrangement is shown in Figs. 14, Plate 5. Fouling-bars, which are depressed by passingvehicles, are subject to severe wear and tear, as they receive what is practically a blow fromevery wheel of atrain passingover them,and this wear andtear has beenfound to be minimized by the provision of cushion springs, spiral in form, as shown in Fig 15, Plate 5. A form of point-rodjoint designed by theAuthor is shown in Fig. 13, Plate 5 ; this has been used for some years, and where put down has entirely preventedbreakage at the joints. ,It will be seen that each half of the joint is both male and female, and that whenbolted together thestrain on thejoint, which occurs when the rod is moved, is taken up by the dovetailing of the joint itself, the function of the connecting bolts beingsimply to keep the two parts of the joint together. Thestandard for point-rodding is a solid roundrod, laid in 16-foot lengths upontravelling rollers 5 inches in diameter,and jointed as just described. TheAuthor has designed a roller for use with a T-sectionpoint-connection, in which the rod rolls on its flanges, the web hanging loosely in a groove on the roller; thishe purposes tryingin actual use. Theprinciple of thejoint shown in Fig. 13 will be applied to the T-section rodding. Power working of signals has made considerable progress within thelast few years, butin the Author’s opinion it is not likely to come into general use, nor to supersede to any very large extent the ordinary manual system ; its first cost is generally greater than that of a manual system, while the cost of maintenance is certainly not less. The chiefscope of its usefulness would appear to be in

largestation-yards, where it hasundoubted advantages. Among these are the reduction in the size of the signal-box and in the length of the interlocking-frame ; the greater ease of manipulation ; the fact that fewersignalmen arerequired ; andthe absence of rodding and wires above ground, which under the most favourable conditions offer more or less obstruction toshunters and others workingupon the line. TheGreat Western has at presentthree installations of power working, all of which are operated entirely by electricity. The chief considerationgoverning the selection of an all-electric system in preference toany other system of power working is that at most large centres (and it is in such places that power working will be most frequently employed) there is a supply of electric current for lighting or other purposes, which can also be adapted to the power working of signals and points. The installations already putdown are at Didcot North Junction, which contains thirty-one working levers ; at Yarnton Junction with forty-three ; and at Birmingham North-End, where as many as one hundred and eighty-three levers are installed. Further installations, whichwill also be all-electric, will be put down beforelong at Birmingham South-End and at Paddington Station.

AUTOMATICELECTRICAL INDICATORS. It is becoming increasingly the practice to detect facing points by means of electrical detectors in place of mechanical detectors. This is particularly the case where a signal is required to detect a succession of facing points, as the Connections between the signal and the mechanicaldetectors, when multiplied, frequently result in the signalworking very heavily, and sometimes it fails to go

circuit.Where this is outside the home signnl, itsfunction is to hold the block indicator for the block section in which the trxin is standing at “ train on line ” :rind to give the signnlman a visual indication that the train is at the signal. In the case of a train standing at a starting-signal,out of sight of the signalman, the function of the track-circuit is to hold an electric lock on the home signal in the rear, and also to give an indication to the signalman that the train isat the signal. Single vehicles cannot be invariably relied upon to operate the track-circuit, but engines and complete trains do so satisfactorily. Two installations of controlled automatic signalling, using track- circuits, are in use on the Great Western Railway, namely, on the quadrupled lines between Tilehurstand Pangbourne, and on the quadrupledlines between Pangbourne and Goring. Theirgeneral arrangement is shown in Fig. 17, Plate 5. It will be seen that the line between A and C, previously one block section, has been divided into two sections, with electrically worked signals placed midway between A and C, at a point B where there is no signal-box or signalman. Thesignals are controlled bythe signalman inthe signal-box in the rear, at A, subject to a control which is exercised by the trains themselves through the track-circuit. The method of control is as follows :- Thetrack-circuit between thepoints marked B and C when occupied by a train holds the intermediate distant and stop signals at B at danger. The track-circuit between A and B when occupied holds thestarting and distant signals for A at danger. Block- telegraph working is maintained for the section between B and C andthe intermediate stop signal at B cannot be loweredby the signalman at A unless the track-circuitahead is clear andthe signalman at C has given usual block-telegraph permission for the train to approach his station. The intermediate block section A B is therefore in reality an additional block section under the control of the signalman at A. Audible Cnb-Signalling.-Perhaps the most noteworthy development of railway signalling in recent years is the introduction on the Great Western Railway of n system of audible distant signals, which is the invention of some of the company’s own officers. This system possesses the following three distinct advantages over the usual distant semaphore :- It distinguishes distant signalsat night as well as by day. It gives distant signals to the engine-drivers in the cabs of the engines. It dispenses with fogmen.

According to this system of signalling, the “ on ” position of the distant signal isindicated to thedriver by means of asteam- whistle, andthe “ off” position by the ringing of a bell. The whistle and the bell are arra.nged in an instrumentfixed in the cab of the engine and called the “ cab apparatus” (Fig. 16, Plate 5). The whistle is arranged on the principle of the safety-valve, that is, steam is always trying to blow it, but is prevented from doing so by means of a valve held down by an electromagnet. This electromagnet is normally energized by a battery on the engine, and in circuit with it is a switch attached to the plunger of a contact shoe, which is fixed in acentral position upon the underside of the engine.This shoe is normdly 24 inchesabove rail-level. The plunger is capable of being lifted vertically 2 or 3 inches, and is so arranged that it opens the switch in circuit with the electromagnet when the plunger has been lifted inch. On the line, in the centre of the track, and thusin the path of the contact-shoe on the engine, is a ramp, about 40 feet in length, which is an electric conductor, consisting of a steel T-bar mountedon a length of timber.The highest point in this ramp is 4 inches above rail-level, so that the plunger inthe contactshoe in passing over the rampis lifted l$ inch. Theramp is connected electrically with the signal-box, where, by means of a switch, the signalman can, when the line is clear, connect it with a battery,so that theconductor on the ramp is electrically “alive” ; otherwise it is “dead.” When the line is not clear, and the conductor on the ramp is “dead,” an engine fitted withthe audible-signalling apparatus passing over the ramphas the plunger of its contact-shoe raised, and the switch in the circuit of the electromagnet inthe cab apparatus opened, theresult of which is that the whistle begins to blow. If the line is clear, and the signalman has switched the battery on to the ramp so that the conductor is ‘‘ alive,” then, although the switch in the circuit of the electromagnet is open, theextraneous current is picked upfrom the ramp through two polarized relays in the cab apparatus. The armature of one of theserelays short-circuits the switch inthe circuit of the electromagnet, so that the electromagnet connected with the whistle-valve isnot de-energized, andthe blowing of the whistle isin this way suppressed.The other relay closes the circuit of an electric bell,which commences to ring.Either the whistle or the bell, when once started, continues to sound until the driver stops it, by lifting a handle in theone case, or by pressing a push-button inthe other.The switch inthe signal-box, by means of which current is conveyed to the ramp, is connected with the distant-signal lever or an equivalent lever. It a-ill be noticed

that this system of audible signalling is designed so as to comply with the well-known rule in railway signalling that the failure of a signallingfunction shall always produce the on ” or danger ” signal. Thus, in the case of any failure, either of the apparatus in the cab of the engine, or in the ramp circuit, the whistle willbe sounded. SINGLE-LINE,WORKIKG. When permanent-way men are working on single lines and doing any work which is likely toaffect the safety of trains, two flagmen under ordinary circumstances are sent out, one on each side of the work which is in progress, in order to stop any train whichmay approach.The number of men employed forsuch a purpose represents a considerablepercentage of the total number of men employed upon siogle lines, and Mr. (now Sir) James Inglis a few years ago, in his capacity as Chief Engineer of the Great Western Railway, introduced telephones at different points on single lines, worked bp one engine in steam, so that permission to occupy the line might be arranged between the permanent-way men and the traffic staff, the necessityfor flagmen being thus dispensedwith. This idea has been developed considerably and has been applied to lines workedon the electric train-staffand tablet systems. In these cases instruments, which are called ‘‘ occupation instruments,” are fixed at differentpoints along the railway, in each ganger’s length, and an occupation key is provided which is common to all theinstruments. This key is normally locked up in one of the instruments, and when a ganger requires to occupy the line for my purpose, such as for running a trolley or for taking out a rail, he obtains possession of the key with the permission and co-operation of the signalmen at the electric staff or tabletstations onboth sides ; then, whenhe has obtained possession of the key, it is impossible for a staff or tablet to be withdrawn for a train to run over the section for which the occupation has been granted. When workon theline is finished, the gangercan restore the electric train-staff or tablet working by replacing the key in any one of the occupation instrumentsin his length. So successfulhas this systemproved that onlines where it has beenintroduced it hasbeen possible to effect anannual saving of L12 permile in mnintenance. When the foregoing occupation instrument was designed, it was seen that its principle could be usefully extended to the working of trains themselves. Installatious have been provided in which a train carrying a key can be admitted to an intermediate siding on a

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. Proceedings.] RAILWAY-SIGNALLING.BLACKALL ON 167 single line between two staff stations, and when the train has been securely locked in the siding and the key locked in an instrument provided at the siding, the electric train-staff system is restored so that atrain can run over the single line ineither direction. In such a case,when the staff is out for athrough train, it is not possible for the key to be obtained at the intermediate siding to allow the train there to pass out on to the single line again until such time as the staff carried by the through train has been replaced in one of the instruments at thestaff station.

TELEPHOXESAKD TELEGRAPHS. Duringrecent years the telephonehas been very generally introduced in connection with railwa.y-working. For railway purposes it is necessary that a number of stations or signal-boxes should be connected by means of an " omnibus " circuit, each station being able to call up any other station without the intervention of a third party. This system of working is necessary, not only from an economical standpoint, but also onaccount of the importance of beingable to establish immediatecommunication withoutthe intervention of an exchangeoperator. In connectionwith these omnibus circuits the Great WesternRailway has introduced a selective system of signalling, in which any station on an omnibus circuit can call up any other station without thecall being repeatedat other stations on the circuit. The principal advantage of this is that it preventsunnecessary ringing in busysignal-boxes and offices. It has proved very useful, but the apparatus is necessarily rather more complicated than ordinary telephone-apparatus, and requires more skilled attention in connection with its maintenance. For ordinary omnibus telephone-circuits, whereit is not considered that the extra cost of arranging for selective ringing is warranted, a system is being introduced wherein a common battery suffices for bell-ringing and conversation for allstations connectedwith the circuit.This systemhas theadvantage of effecting a considerablesaving in battery power. Superimposingtelegraph- and telephone-circuitsone upon the other is largely resorted to on the Great Western, and some very considerable economies arethereby effected. Practicallyall the telegraph-circuits emanating from Paddington are superimposed on trunk telephone-circuits. In other cases telephones are superimposed on single-wire telegraph-circuits, Thedifficulty which is met in these latter cases is the induction thatoccurs on the unbalanced telegraph- line. This difliculty, however, has been largely overcomeby intro-

Downloaded by [ University of Liverpool] on [15/09/16]. Copyright © ICE Publishing, all rights reserved. 168 BLACKALL ON RAILWAY-SIGNALLIKG. [Minutes of clucing into the receiving circuit of the telephone a condenser of small capacity, which in practice is found,while not too greatly diminishing the volume of the received speech, practically to elimi- nate all disturbing noises due to low-frequency currents induced from other circuits. Telephones can beworked in this way up to a distance of 100 miles, if there are no heavily worked telegraph-circuits on the same poles as those on which the superimposed telephone-circuit is carried, or, where the line is subject to heavy induction, a telephone superimposed on a telegraph-wire may be satisfactorily operated up to a distance of 50 miles. The increasinguse of long-distancetelephones renders the use of copper wire necessary to a greaterextent than was formerly the case, and the system of wiring adopted on the Great Western is as shown in Fig. 18, Plate 5. Arms 4 feet in length are used. The copper wires for the telephone-circuits,with the telegraph- circuits superimposed uponthem, are revolved in the usual way, and are carried at the top of the poles.Below theseare run 400-lb. iron wires for the block and other circuits, in which high conductivity is not a matter of supreme importance ; at the bottom of the pole are carried 67-lb. bronze wires, which are used for the electric repeater and other short and electrically unimportant circuits. The length of the spans is 55 yards, Thereis not much to record in regard to railway-telegraph message-circuits beyond thefact of the gradualdisplacement of the single-needle instruments by telephones. Thereare still a largenumber of single-needleomnibus circuits in operation, and some belatedimprovements in regard tothe working of these circuitsare now beingintroduced. In 1875 thelate Professor Ayrton pointed outthat the bestresistance to give tothe instrumentsor relays of omnibussignalling circuits was to be found from the formula R = L/P, where L is the total resistance of the telegraph-line, and P is the number of instruments on the circuit.This does notappear to have been properlyappreciated, as the standard resistance of the single-needle instruments in use on theGreat Western has been, until recently, not less than 130 ohms, andthis may be the same elsewhere. Takingindis- criminately 1,000 of the totd number of such instruments in use, their bestaverage resistance, according tothe ruleenunciated, should be about 35 ohms. The matter is one of some importance, as in practice, by adopting Professor Ayrton’s formula, insulation

Journal of the Society of Telegraph Engineers, vol. 4, p. 85.

difficulties are reduced ; and moreover in those cases where telephones are superimposed on single-needle circuits, the lowering of the resistance and consequent reduction in the impedance gives much better telephonic results.

The features of modern signalling described in. this Paper differ probably very little from the methods employed by companies other than the Great Western Railway, as the known requirements of the Board of Trade, based largely, no doubt, on observation of the best signalli.ng practice, tend very much to uniformity. In conclusion, the Author desires to express his view that railway signalling has not yetreached its ultimate development, and that the railway-signalengineer, who shouldhave at his command all the many schemes and devices which the experience of many years has provided for him, must regard them merely as tools, to be used or thrownaway as necessity may demand.Holding this view, the Author has had no hesitation in recommending the Great Western Railway Company to adopt new methods whenever such appeared to him to be an improvementupon the old. Standardization is very desirable, but new standards should be substituted for old whenever an improvement is found to be economically possible.

The Paper is accompanied by sixteen tracings, from which Plate 5 and the Figure in the texthave been prepared.