RAILWAY TELEPHONES1072

WHILE to some extent the communicationrequirements of railway companies are met bytelegraph and telephone systems generallysimilar in principle to those employed in thepublic service, the special nature of problemswhich may concern, for example, the control oftraffic in densely populated areas or over dis-tances of hundreds of miles has necessitatedthe development of systems providing facili-ties not demanded elsewhere. Railways wereamongst the first commercial undertakings tomake extensive use of the telephone, and theirprogress in this respect has been no lessmarked than in others. Every opportunity hasbeen seized to make the fullest possible use ofinvention in the world of communication engi-neering, and the equipment employed to-daybears witness to the skill of the telephone engi-neer in meeting a diversity of unusual require-ments.

The Handling of Railway Traffic requiresSpecial Telephone Systems

It is convenient to classify railway telephonesunder two headings : (1) those employed forgeneral commercial purposes, and (2) thoseused chiefly by the operating staff. Of theformer almost every known type will be foundin service, the range embracing small “domes-tic” telephones, push-button intercommunica-tion sets, and magneto, central battery and au-tomatic instruments. The design and opera-tion of all these are referred to in other sectionsof this work, and we may therefore pass at onceto the types specially evolved for railway serv-ice ; namely, telephones providing facilitieswhereby the movements of trains and the gen-eral handling of traffic may be effectively con-trolled.

How Communication between DistantStations is achieved - Omnibus Circuits

In the provision of a railway intercommuni-cation system the problems introduced are unu-

sual, chiefly by reason of the respective posi-tions of the various points to be served.Whereas a public telephone network is builtup of a number of exchanges or switching cen-tres with individual lines radiating from eachexchange to the local subscribers, a railwaysystem must provide for the interconnectionof stations, signal cabins, etc., situated at in-tervals along the track. For obvious reasonsthe erection of switching equipment at certainpoints and the connection of each station andsignal cabin by an independent line would betoo costly. We therefore find that the principleof party line working is adopted, the lines be-ing commonly termed “omnibus” circuits.

Operation of Omnibus CircuitsFor communication between a limited number

of points not widely separated the ordinarymagneto telephone may be employed, and sev-eral such instruments bridged across a com-mon pair of line wires. The use of a ringingcode then permits each station to call any ofthe others. Again, battery ringing sets may beused, in which case a high resistance relay isusually inserted in each instrument and the lo-cal calling bell operated from its contacts. Coderinging systems of this kind are extensivelyapplied to serve groups of points, such as sta-tion offices, staff quarters, signal cabins, plat-forms, goods sidings, etc.

Using Omnibus Circuits for Traffic ControlPurposes

A step further is reached when we considerthe control of the operating staff and the provi-sion of means whereby immediate connectionmay be established between a central controloffice and every point on the railway systemwithin an area embracing both main and branchlines and extending over hundreds of squaremiles. Comparatively simple and inexpensivemethods have been devised, the most commonconsisting of the extension of the omnibus cir-

RAILWAY TELEPHONESBy S. FLINT

Fig 1. - WAYSTATION TELEPHONE

FOR OMNIBUS CIRCUIT TERMINATED

AT A CONTROL OFFICE

Telephone shown open forinspection. In this type, carbonlightning arresters are included anda lock is provided to rpeventinterference by unauthorisedpersons. (G.E.C.)

1073RAILWAY TELEPHONES

cuits to the central point andtheir termination on one ormore operating positions,built in the form of desks, fromwhich " controllers " may su-pervise the movement of anyor all classes of traffic.

This further use of the om-nibus circuits for control pur-poses is very economical inthat the expense of erectingadditional line wires is avoided.It would be impracticable,however, to employ callingbells in the control office, par-ticularly where large numbersof circuits are concentrated,

and it is therefore usual forthe lines to be terminated onvisual or lamp indicators oneach controller's desk. Suchdevices do not satisfactorilyindicate codes as normallyemployed for interstationcalling, and it will thus beseen that a means must beprovided at each way-sta-tion for operating the controlindicator, and only when at-tention of the controller is re-quired.

For this purpose reversebattery calling is adopted,each waystation telephonebeing equipped with an ad-ditional ringing key for sig-nalling to the control office.A typical instrument of thistype is shown in Figs. 1 and2.

The Dual Purpose Tel-ephoneFrom the circuit diagram

in Fig. 3 it will be seen that

Fig. 2 - WAY-STATION TELEPHONE FOR OMNIBUS

CIRCUIT TERMINATER AT A CONTROL OFFICE

The calling code and full operatinginstructions are added on a printed cardbeneath the two keys. (G.E.C.)Fig. 3 (right) - CIRCUIT DIAGRAM OF WAY-STATION TELEPHONE

RAILWAY TELEPHONES1074when the "code" ringing key is depressed posi-tive battery is applied to Line 2 and negative toLine 1. The bells in all other way-station instru-ments will be operated, and, according to thecode transmitted, only the wanted station willanswer. If the “control” key be depressed the

polarity of the ring-ing battery appliedto the line will bereversed. At thecontrol office a po-larised relay isbridged across theline and is so con-nected that it willrespond only topositive battery onLine 1 and negativeon Line 2. This re-lay in turn operatesthe calling lamp orindicator on thecontroller's desk.At the control office,therefore, signals arereceived only when theappropriate way-station key is de-pressed.

When ringing

current is applied to the line at the control of-fice or any other waystation the high resist-ance relay shown will close the contacts of thelocal bell circuit. Irrespective of the nature of acall from a way-station, i.e., whenever eitherthe “code” or “control” key is depressed, the

bells will be operated at allother stations unless the re-lay in each instrument is po-larised. It is therefore commonpractice to equip all sets onthe circuit with polarised re-lays if a large number of callsis made from the waystationsto control. If this were notdone every telephone bellwould be operated for eachcontrol call made, causing un-necessary noise and a wasteof local battery current.

ConnectingOmnibus Circuitswith OrdinaryRailway Intercom-municationSystems - RailwaySwitchboards

Although omni-bus circuits have beenconsidered independently,it must not be assumedthat the points they servenever require connectionwith commercial and otherrailway departments pos-sessing ordinary manualor automatic intercommu-nication systems. At largerailway centres it is oftenfound necessary to pro-vide facilities for the inter-connection of circuits ofmany different classes, a

requirement which is onlysatisfied by the use of en-tirely special types ofswitchboards. Such manu-ally operated boards are

Fig 4. - TYPICAL C.B. SWITCHBOARD FOR RAILWAY

SERVICE, EMPLOYING PROTECTED DISC INDICATORS

Capacity is provided for the addition of further circuitswhen required.

1075RAILWAY TELEPHONES

generally unique in that theyare designed to meet specificlocal conditions, and it is safeto assume that there are notwo alike. For the purpose ofillustration, however, a typi-cal example is shown in Fig.4.

Typical EquipmentThis switchboard clearly

indicates the variety of lineswhich may be concentratedat one switching point. It isof the central battery type,employing visual disc indica-tors throughout, and isequipped for:

40 C.B. extensions (localoffice lines).

10 junctions to a publicautomatic exchange.

10 Railway omnibus lines.5 Railway trunks.

Owing to the miscellane-ous nature and additionalquantity of apparatus neces-sary for a switchboard of thiskind it is not possible tomount all components at therear as in the case of an ordi-nary private branch ex-change. Provision is thereforemade for certain relays, coils,etc., to be assembled on anauxiliary rack which may beerected nearby and con-nected to the board by multi-ple cable. The apparatus rackassociated with the board de-scribed is shown in Fig. 5. Onthis also are terminated the in-coming lines via the usualprotective devices.

The Circuits and theirOperation

With regard to themethod of terminating vari-ous lines and the operat-ing procedure when deal-ing with calls over the dif-ferent circuits, we canagain refer to Fig. 4. The ex-tension line jacks will beseen at the bottom of thetwo panels, the associatedindicators appearing at thetop. These standardC.B. extension circuits op-erate in the regular manner,and supervision is givenby disc indicators mountedin the face of the keyboard.The next group of appara-tus consists of ten jacks,indicators and keys forlines to the public auto-matic exchange. Here it be-comes necessary, when theoperator wishes to call apublic service subscriber,to insert in the circuit thedial seen on the right. Thisis effected by depressingthe key associated with theline taken into use, the keybeing restored after the re-quired number has been di-alled. A typical connectionis shown, by the two cordsand plugs in use, betweena local extension and a pub-lic subscriber, the dialling

key being depressed readyfor the operator to dial thewanted party.

In the middle of the left-hand panel are terminated

five railway trunks, special long distance linesemployed for the direct connection of impor-tant centres. Here again a key is associated witheach indicator and jack, but in this case for ap-

Fig 5. - APPARATUS RACK FOR SWITCHBOARD

IN FIG 4.This may be erected either in theoperating room or some distance away,connection with the switchboard beingeffected by mutliple cable.

RAILWAY TELEPHONES1076plying ringing current to the line, the nature ofthe trunk termination not permitting the use ofA.C. ringing from the cord circuits. Above thesekeys is an auxiliary strip of jacks whereby atnight the trunks may be left plugged throughto other circuits and the corresponding switch-board calling apparatus entirely disconnected.The usual night switching keys in the cord cir-cuits appear at the top of the board.

The remaining lines, each with indicator, jackand ringing key, are omnibus circuits of the typealready described. Details of their terminationon the switchboard are shown in Fig. 6.

Calls over Omnibus CircuitsThe line relay is of the polarised type which

operates only when the appropriate key at away-station instru-ment (Figs. 1-3) isdepressed. Thus,when a call to theexchange is made,the line relay is mo-mentarily energisedand, at its contacts,completes the indi-cator cir cuit. Thisimpulse of currentthrough the indica-tor coil results inthe release of thedisc signal, whichthen remains displayed to the switchboard op-erator. When an answering plug is inserted intothe line jack the indicator disc is automaticallyrestored (such indicators being known as the“plug-restored” type) and the speech circuitbetween way-station and operator is complete.A “hold” coil is bridged across the line to givethe same condition for supervision as when acall is made from a C.B. extension telephone.Condensers prevent the flow of exchange bat-tery current (fed from the cord circuit) over theomnibus line.

After the desired switchboard connectionhas been established and the conversationends, the key at the waystation telephone is

again depressed to give a clearing signal to theoperator. The polarised line relay operates, butnow, since the answering plug still remains inthe jack, the line relay contacts complete thecircuit of the "clear" relay. The latter is ener-gised, and in turn removes the "hold" bridge,thus giving a clearing signal in the appropriatecord circuit. The "clear" relay at the same timeconnects its own winding to earth via the tipspring of the line jack and the cord circuit. Ittherefore remains operated and maintains theclearing condition until the plug is removed fromthe jack.

To call a way-station on an omnibus circuitthe operator first plugs into the correspondingline jack to ascertain if any other call is inprogress (between two waystations). If the cir-

cuit is free the appro-priate code is trans-mitted by means ofthe ringing key, socalling the wantedstation. The clearingdown of the connec-tion is effected asbefore.

Other Types ofSwitchboards

The design of arailway switchboardat which different

types of circuits are concentrated dependschiefly on the system employed for the inter-connection of the local extensions. In somecases local magneto systems are used, theswitching equipment then being somewhat sim-pler. Again, if central battery working is fa-voured, the calling devices may consist of lampsignals in place of the visual indicators shownin Fig. 4.

A two-position C.B. lamp signalling switch-board for railway service is illustrated in Fig. 7,apparatus being provided here for 60 exten-sions, 10 public automatic exchange lines, 10railway trunks, and omnibus circuits. Capacityfor the future addition of new circuits is given

Fig 6 - TERMINATION OF A RAILWAY OMNIBUS CIRCUIT

AT A C.B. SWITCHBOARD EMPLOYING PLUG-RESTORED

DISC INDICATORS (24 VOLT SYSTEM)

1077RAILWAY TELEPHONES

by the spaces covered by blank strips (or dum-mies). It will be seen that while the extensionlines are distributed - thirty per position - theother lines appear on both positions, so as togive each operator direct access. On this ac-count an extra strip of lamps is associated oneach position with the public exchange lines inorder to give “visual engaged” indication. A“visual engaged” lamp on position 2 glows foreach circuit taken into use on position 1, andvice versa.

While only two typical small manually oper-ated switchboards have been illustrated, it willbe understood that at very large railway cen-tres the local telephone system only may berequired to serve up to a thousand lines or more.

Fig 7 - TWO-POSITION RAILWAY SWITCHBOARD OF THE C.B. LAMP

SIGNALLING TYPE

This type gives the greatest possible uniformity in face equipment forextension lines, railway trunks, omnibus circuits and junctions to thepublic automatic system

The equipment employed in such cases,whether of the manual or automatic type,follows standard practice, and, in general,does not differ from that used for publicservice except in the provision made forthe termination of railway trunk and omni-bus circuits on special operators' posi-

tions.

Control Lines withSelective Ringing

While omnibus circuits asdescribed are widely em-ployed in many areas, they

possess certain limitations which haveled to the development of still furthersystems in which the number of way-stations served by one pair of wires isgreatly increased. Difficulties arise in theuse of an ordinary omnibus circuit heav-ily loaded with telephones, parcularly inthe matter of codes. As the number ofwaystations increases the more involveddoes the code become. Traffic is heavier,

and, as a result, the bells at all sta-tions are ringing almost continually.In a number of areas selective ring-ing systems are therefore employed,specially designed for serving largegroups of waystations by one com-mon two-wire circuit. Such installa-tions are used almost exclusively for

control purposes, and on this account do not ordi-narily provide facilities for inter-station calling.

In the latest system to be adopted the selectionand calling of a way-station from the control officeis effected by an ordinary automatic telephone dial.At each station a selector responds to impulses trans-mitted from the dial, and in this way a method isobtained whereby the bell is rung at the wanted sta-tion only.

Reverse Current ImpulsingIn automatic telephony signalling is accomplished

merely by interrupting the line current. This is effi-cient if the line resistances and capacities are of rea-sonably low values, but cannot satisfactorily beemployed for railway control circuits which mayextend over fifty miles or more and serve in some

RAILWAY TELEPHONES1078cases up to ninety points. Selection is therefore ob-tained by reverse current impulsing, in which eachimpulse is represented by a reversal of the directionof the normal line current instead of by the usualbreak. This system of impulsing practically over-comes all troubles liable to be caused by wire-to-wire capacity.

Station SelectionIt will be seen in Fig. 8 that the line at the control

office is connected directly to the operator's tel-ephone. When the dial finger-plate is rotated in aclockwise direction (preparatory to its release andthe actual impulsing), contacts A are operated, auto-matically locked, and the line battery placed acrossthe line. The dial impulse springs control a relaywhich, at contacts B, connects the line battery in adirection depending upon whether the relay is oper-ated or released. Thus, when the dial finger-plate isreleased, the current flowing over the line is reverseda number of times according to the digit dialled.

The telephone at each way-station (Figs. 9 and10) contains a polarised relay which operates to thecurrent received before impulsing commences andreleases at each reversal. This relay in turn controlsthe stepping of a rotary selector of the type shownin Fig. 11 (a local battery being employed with eachinstrument for operating the selector and associatedrelays).

Each station is allot-ted a two-digit numberand is made to respondto this number only bythe setting of strap con-nections inside the in-strument. Thus, in call-ing Station No. 73, thedigit 7 will first be di-alled, but, although allselectors will bestepped to position 7,at only those stationshaving 7 as the firstdigit of their numberwill .a discriminatingrelay be locked andthe instrument pre-pared to respond tothe second digit.

When this is dialled - in this case 3 - all selectors willstep to position 3, but only at Station No. 73, wherethe discriminating relay is operated, will the strapconnections be such that the call bell is rung. Thebell will continue to ring, and superimposes on theline a tone generated by the vibration of the arma-ture. This is fed to the controller's telephone, andceases only when the way-station receiver is liftedand the bell disconnected. The speech circuit is thencompleted by the controller depressing a key tooperate contacts A and remove battery from the line.The selectors at all stations are thereupon restoredto normal.

Common CallsFor the transmission of time signals and general

and emergency instructions to all way-stations aspecial number is dialled which causes all bells toring simultaneously. Ringing tone is fed to the linefrom every instrument, so that when the tone re-ceived by the controller finally ceases he is awarethat all way-stations have answered and are listen-ing. This feature is sometimes termed an automaticroll call.

Control Office EquipmentWhen one circuit demands the continuous atten-

tion of a controller, and when, in such circumstances,his telephone is always across the line, calling facili-ties at the way-stations are unnecessary. If, how-

Fig 8. - SIMPLIFIED CIRCUIT DIAGRAM ILLUSTRATING THE METHOD OF CALLING A WAY-STATION INA SELECTIVE RINGING TRAFFIC CONTROL SYSTEM

Any number of stations up to a total of ninety may be servied by the same pair of linewires. While such systems are normally employed for communication with the controloffice only, facilities for inter-station calling may be given by providing dials also at theway-stations.

1079RAILWAY TELEPHONES

ever, several lines areterminated at the con-trol office, or theamount of trafficdoes not warrant thefull time services of acontroller, each way- station is equippedwith a calling key. Thedepression of this keyapplies battery to the

line and operates a polarised relay to give a callingsignal at the control point. Here the equipment mayconsist of special desks fitted with lamps, keys andtelephones, the vertical face of the desks bearing amap of the railway network under control. For thetermination of a single selective ringing circuit a moresimple arrangement is used.

Connection to Omnibus LinesThe points served by a main railway control cir-

cuit do not always include stations on branch orspur lines with which communication is not oftenrequired. The selective ringing system therefore pro-vides facilities whereby the control operator mayautomatically effect the connection of the main cir-cuit with a distant omnibus circuit ordinarily givingintercommunication between local points only on abranch line. For this purpose a code ringing repeateris installed at the most convenient way-station wherethe two circuits meet. This may be many miles fromthe control office.

Fig 10 - WAY-STATION TELEPHONE

On the numbering panel will be seen the strap connectors for making the instrumentrespond only to its particular two-digit number

Fig 9. - COMPLETE

W A Y - S T A T I O N

T E L E P H O N E

I n c o r p o r a t i n gSelector UnitIn addition to the keyprovided for callingthe control office, anauxiliary “speak” keyis fitted whereby thetransmitter may bedisconnected andreception improvedover exceptionallylong lines.

RAILWAY TELEPHONES1080

The repeater con-sists of a small groupof relays whichfirstly effect the con-nection of the twolines when a predeter-mined number is di-alled from the controloffice. The next op-eration - the trans-mission of a signallingcode over the omni-bus circuit-is carriedout in an ingeniousmanner. The onlycalling device at theservice of the control-ler is the dial used forstation selection onthe main circuit,while the code to besent over the omni-bus circuit must con-sist of a particularcombination of“long” and “short”signals for calling theattention of thewanted waystation.The repeater is there-fore arranged to ap-ply battery to theomnibus line accord-ing to the number andlength of the subse-quent trains of im-pulses transmittedfrom the control of-fice. The speed of thedial being ten im-pulses per second,the sending of thedigit “0” willcause the repeaterto apply battery tothe omnibus circuitfor a period of onesecond. Similarly a“short” signal (ap-proximately one-

Fig 11. - WAY-STATION SELECTOR

The arc of contacts is swept by a set of “wipers” operated by aratchet wheel and pawl. Each impulse of current applied to thedrive results in one step by the wipers. The restraining pawl ispulled out of enggagement when the release magnet is erengised.A return spring is would up as the wipers advance and, when therestraining pawl is withdrawn, returns to normal

third of a second) will beapplied when the digit“3” is dialled. Thus, bydialling a combination ofdigits “0” and “3” the re-peater is caused to trans-mit the way-station codeover the local line.

At the end of the callthe omnibus circuit isdisconnected at the re-peater by the dialling ofa further predeterminednumber. The completecontrol code of aparticularway-station ona distant omnibus line,whose local calling sig-nal is “long-short-long”,would therefore consistof three parts, such as59-030-5, where 59 isthe coupling code, 030the station-ringing code,and 5 the uncouplingdigit.

The connection of thetwo lines at the repeatermay also be effectedfrom any way-station onthe omnibus circuit, anauxiliary control key be-ing provided on each in-strument for this pur-pose.

We are indebted t oMessrs. The GeneralElectric Co. Ltd. for sup-plying many of the illus-trations in this article.