British Signalling – What the driver sees v2 · PDF fileOne of a series of papers originally published as pages on RTWP and updated for RTW. Background Paper No.1 Basic Railway Signalling

Railway Technical Website BackgroundPaperNo.1

BritishSignalling–Whatthedriverseesby

PiersConnor1

IntroductionThis page describes the types of signals seen on British railways and their meanings.Semaphoreandcolourlightsignalsareincluded.

SemaphoreSignalsDuring the 19th century a system of mechanically operated semaphore signals wasdeveloped for Britain's railways. Although there were many different and independentrailwaycompanies,bytheearly20thcentury,signalsweregenerallystandardised,butwithsomevariationsinstyleandappearance.Manysemaphoresignalshavesurvivedtothisday,although theyarebecomingrarer.However, therearesomeexcellentexamplesstill tobeseenontheheritagelinesoperatedbypreservationgroupsalloverthecountry.

SemaphoreSignalPartsFirst,adiagram(left)ofasemaphoresignalanditsmainparts.Thesignalisnormallyplacedontheleftsideofthetrackwiththearmdirectedovertheoffside.Thestandardarmisredwithawhiteverticalband, althoughsome older signals were plainred.Toallowthesignalindicationtobe seen at night, the arm is fittedwith two lenses, duplicating theindicationdisplayed. The lenses areilluminated from behind, originallybyoillamps,laterbyelectriclamps.

The signal is mounted on a signalpost, originally wooden but laterlattice steelwork, pressed steel, oldrail, and concrete appeared atvariousplaces.Somerailwayscouldbe recognised from the design oftheir signalposts, theones from theVictorianerahavingelaboratefinialsandotherattachments.

Signal posts were often tall, so thatthesignalcouldbeseenclearofenginesmokeandfromadistance.Itwasalsointendedthattheguardcouldseethesignalfromtherearofthetrainasitwaspartofhisdutiestochecksignalindications.

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OneofaseriesofpapersoriginallypublishedaspagesonRTWPandupdatedforRTW.

BackgroundPaperNo.1 BasicRailwaySignalling

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Thepostisnormallyfittedwithaladder,originallytoallowaccesstotheoillampsandnowretainedformaintenance.Thepost isoftenprovidedwithatelephonelinkedtothesignalbox.Thetelephoneiscontainedinasmallboxwithblackandwhitediagonalstripesonthecover.Thedriverorothertraincrewcanusethistoalertthesignalmanofthepresenceofthetrainandtoenquirewhythesignalremains"atdanger",aswesayinthetrade.

The armof the signal is displayed in ahorizontalposition to show the "stop"or "danger"indication.Theredlensisilluminated.Theindicationtothedriveris"stop"untileitherthesignal indication gives "proceed" or he is given verbal authority by the signalman toproceed.

Togivea"proceed"indication,thesemaphorearmisraisedtoanangleof45°asshowninthediagram.Themovementofthearmcausesthegreenlenstoreplacetheredlensinfrontofthelamp.A"proceed"indicationtellsthedriverthathemayproceedatnormalspeedforthatsectionof line, subject toanyspeedrestrictionsdisplayedandaccording to thespeedlimitoftheparticulartypeoftrainheisdriving.Unlikemanyothercountries,Britishsignalsdonotgiveaspeedindicationtothedriver.

Somesignalsshowed"proceed"byloweringthearminsteadofraisingit.Thistypeofsignaliscalleda"lowerquadrantsemaphore", (shownasan inset in thediagram)asopposedtothemoreusual"upperquadrant"type.ThelowerquadranttypewasmuchfavouredbytheGreatWesternRailwaybuteschewedbyothersfollowinganaccidentcausedbyastopsignalshowing a proceed indication because it droopedwith theweight of snow resting on thearm.TheGWRmaintainedtheyhadneverhadanaccidentcausedbythistypeofsignalsotheyweren'tabouttochangethemnow.Anyway,theydidn'twanttheexpense.

Onefinalpartofthesignalmechanismisthebalanceweight.Itislinkedtothecablewhichoperates thesignal.Thecable,ofcourse, isconnectedto the lever in thesignalboxwhichoperatesthatparticularsignal.Thepurposeofthebalanceweightistopullbackthesignalwirewhentheleverisreplacedintheframebythesignalman.

TypesofSemaphoreSignalThe following series of diagrams, with descriptions,showsthevarioustypesofsemaphoresignalsseenintheUK.

A Home Signal or Starting Signal (left) is the stopsignaldescribedabove.Itisplacedattheentrancetoablockand,whenshowing"stop",thetrainisforbiddento enter the block. When a signal shows a stop orother restrictive indication, it is said to be "on". Asignalshowingaproceedindicationissaidtobe"off”.

Traditionally, ata station,each trackwouldhave twostopsignals.One,protectingtheentrancetotheblock,wascalledtheHomeSignal.Theother,protectingtheexittowardsthenextstationorsignalbox,wascalledtheStartingSignalorStarter.

As mentioned above, this is a stop signal showing aproceedindication-itis"off".Thetrainmayentertheblock at normal speed. In effect, this means themaximumspeedapplicable to thissectionof lineandthetypeoftrain.

To give advancedwarning of the indication of a stopsignal, a "distant" signal is sometimes provided(left).Thisoperatesinthesamewayasthestopsignalbutgiveseithera"caution"indication(it issaidtobe



"on"),shownontheleft,oraproceedindication,ontheright.Ifthedistantis"on",ayellowlightshowsatnight.Thedistantsignalshowing"on"tellsthedriverthatthenextstopsignalisalso"on"andthathewillhavetostopthere.Thedistantsignalwas,ifpossible,located¾mile (1200 metres) before the stop signal. A single distant signal will often provide awarningforbothhomeandstartingsignalsatastation.

The distant shows a yellow (on) or green (off) light at night. Remember that the distantsignal normally refers to more than one consecutive stop signal ahead. Thus, when thedistantisoff,thedriverknowsthatalltherelevantstopsignalsareofftoo.Eachstopsignaldoesnothaveitsowndistantsignalinrear.

Where blocks were short orstations close together, the distantsignal was often placed on thesame post at the previous stopsignal (diagram left). The drivernowhas two indications,one fromthe stop signal protecting theentrance to the block, the otherfrom the distant for the next stopsignal. To avoid confusion, if thestop signal is "on", the distantwillalso be "on", even if the next stopsignal happens to be "off". This isachievedbylinkingthetwosignalsmechanically - a system known as

"slotting".Atnight,thedriverwillseetwolights,aredoverayellow.Theredalwaystakesprecedence.

At a signal post with home and distant together, there will be occasions when the blockimmediatelyahead is freeandthetrainmayenterbut thenextblockmaybeoccupied. Inthiscasethedriverwillseethestopsignal"off"andthedistant"on"asshownhere.Atnight,hewillseegreenoveryellow.Thisshowshimhemayproceed intotheblockbut that thenextstopsignalis"on"andhemuststopthere.

Thethirdindicationforastopanddistantsignaliswherebothare"off".Thedriverisbeingtoldboththisblockandthenextarefreeandhemayproceedatnormalspeed.Atnighthewillseegreenovergreen.

In somevery restricted locations, a repeating signalis provided, often referred to as a "banner"signal (diagram left). It is a black band on awhitedisc which repeats the position of the semaphorearm.

JunctionSignalsAtjunctions,ithasalwaysbeenthecustomintheUKtoshowadrivertherouteset,notjusttoshowthepermittedspeedasusualelsewhere.Insemaphoresignalling,itwasnormaltosplitthesignalsasshownbelow.

BasicJunctionSignals

Seebelow(a).Thispairofstopsignalsprotectsapairofdivergingroutes.Thelowersignalis "off", indicating that thediverging route to the left of the two is set and lockedand thetrain may proceed along that route. The position of the two semaphore arms was often



arrangedsothatonewaslowerthantheothertoindicateaslowerroute,usuallybecauseofacurve.

SplittingDistants

See left (b). Junctionsignals also have anequivalent distant setup. These are alwaysreferred to as "splittingdistants". They provideadvance warning of theposition of the junctionsignalsand theyoperatein the same way asregulardistants.

Therearealsocombinedversions of the splitting

distant (c)wherea stopsignal isplacedover thedistant for themainroute.Eitherof thedistantscanonlyshow"off"ifthestopsignalisalso"off".

Theabove list isbasicallyall there is toBritishmain linesemaphoresignalling.Ofcourse,thereareadditionalsignalsusedforshuntingandotherlocaloperationsasdescribedbelow.

SubsidiarySignalsTheuppersignalshown in thisdiagram(left)wasatypical shunt signal, used to allow movements intoand out of a siding. It was a miniature semaphoresignal with red and green lamp indications. Bothupper and lower quadrant varieties werecommon. The signal was placed on a short post atgroundlevelorwasattachedtoasignalpostbelowanormalstopsignal.Subsidiarysignalsarethoseusedfor restricted train movements. These sorts ofmovementstakeplacewithinasingleblockorinandout of sidings. Theproceed indication to the driverwasarestrictedmovementsaying"proceedatsuchaspeed that you are able to stop short of anyobstruction".Thereareanumberofdifferent typesofsecondaryofsubsidiarysignalsinuseand,likethe

mainlinesignals,therewerelotsofvarieties.Themostcommonaredealtwithhere.

Alaterversionofthesemaphoreshuntsignalwasthistype,withasolidredarm(aboveleft,lower),whichappearedfrom1925.Itshowedawhitelight,notred,when"on".Itwasalsousedfor"callingon"and"warning"movements."Callingon"referstotheadvanceofatrain

into an occupied section and was often used atstations for coupling purposes. A "warning" signalwas used to advise that the overlap beyond a stopsignal is occupied. They were also used for "shuntahead"movements-movementswhichhavetopassthemain signal for shunting purposes, normally tothen 'set back' into a siding etc. The letter "S"wasrevealedwhen the armwas "off", aswas the letter"C"or"W".

Many semaphore shunt signals were replaced by disc signals (above left) to improvevisibility. The operation was the same and the arm was usually positioned on a whitedisc.ManyofthesecanstillbeseenintheUK.



ColourLightSignalsThefirstcolourlightsignalsappearedintheUK in the 1920s, simply as copies ofsemaphores. However, with the need forbetter track usage and higher speeds, theconcept of multi-aspect signals arrived,giving the driver advance warning of theconditionofseveralblocksahead.

A simple 2-aspect colour light signal (left)which would act as a replacement for asemaphore stop signal. The red aspect isshownhere.Theotheraspectisgreen.A2-aspectdistantsignalwouldhaveyellowand

greenaspects.Thewhiteplatebelowthesignalwilldisplayanidentificationplateusingthereferencelettersofthecontrollingsignalcabinandthesignalnumber.

The3-aspectsignal(shownabove,right)wasdevelopedtoallowhigherspeedsandshorterblock sections to accommodate more trains. The three aspects are red, yellow and

green. The red indicates stop, the yellowindicates thatonlyoneblock sectionaheadisclearandthenextsignalwillshowastopaspect.Thegreenindicatesthatatleasttwoblocksaheadareclear.

The 4-aspect signal (left) is a furtherdevelopmentofthemulti-aspectconcept.Inadditiontotheredaspectatthebottom,thissignalshowsasingleyellowtoindicateoneblock ahead is clear, a double yellow (oneabove theotheras shownhere) to indicatetwo blocks ahead are clear and green to

showatleastthreeblocksaheadareclear.

At certain location where space is limited, a 4-aspect signal can be arranged as shownabove.Theredaspectisplacedtoonesidetoreducetheheightofthesignal.

4-AspectOperatingSequenceAsshowninthediagramtothe left, inanareawhere4-aspectsignallingisinusethesequencefor the four signals protecting the four blocksbehind a train would be red protecting theoccupied clock, then single yellow, doubleyellowandgreen in the following threeblocks.Theviewhereisforeshortenedforthediagram.The signals in a 4-aspect installation will beabout 750 to 850 yards (686 to 777 metres)apartinanintensivelyusedareaandupto1400yards(1280metres)apartinahighspeedarea.The signals are shown without overlaps. Thesequence for3-aspect signalling (coveringonlythree blocks) would be the same but withoutthe double yellow aspect and its associatedblock.



RouteSignallingSignalling in the UK has alwaysused the principle of "routesignalling" as opposed to the"speed signalling" philosophyadopted by many European andUS railways. This means thatdrivers of a British train will beshown which route a train willtakewhenitproceedspastasignalprotecting a diverging junction,seediagram left.Thespeedof thetrain will be a matter for thedriverobservingseparaterulesorfixed speed limit signs along thetrackside. The "speed signalling"system shows the driver whatspeedhistrainmustdo,regardlessof the route it will take. Theinterlocking of the signal at thejunction ensures that the speed

aspectsshownareinaccordancewiththerouteset.TheresultoftheUK'suseofthe"routesignalling"philosophyisthatsignalsdisplaysemaphorearms(asdescribedabove)orlightswhichindicatetheroutesetasshownhere.

The route is indicated by a line of five white lightswhichcorrespondtotheapproximatedirectionoftherouteset.The lightsareknownas"a feather".Theywillonlylightupwhentherouteissetandlockedandthesignalisshowingaproceedaspect.Iftherouteisset for the track regarded as themain route ahead,the signal will only show a proceed aspect for thisroute. The "feather" will only appear to indicate adiverging route. Most examples of this signal havefivewhite lightsbut three lightsareusedbyLondonUnderground.

It is possible to show up to seven routes with thistypeofsignal.Theroutestraightaheadwilljustgetaplaingreenoryellow(s)whilethethreeroutestotherightorleftwillgetthegreenoryellowaspects,plusa"feather". Inmanyareas, thediversityofroutesorsighting restrictions do not allow the provision offeathers. In these cases, a number or letter(s) isshowntothedriverwhenthesignalclears.Thiswillindicatetherouteset.

There are still some colour light junction signalswhich do not have a "feather" but repeat thesemaphore "splitting distant" philosophy. At adiverging route, two signal heads, side by side, areprovided. One shows the main route, the other thedivergingroute.

The approach to some junctions is speedcontrolled.Thesignalshowsarestrictiveaspectuntilthetrainhasapproachedtowithinadistancewhichhasforcedthedrivertoreducespeed.Thereareseveralvariantsonhowthis



is put into practice. For a complete description, see the excellent web pageJunctionSignallingbyCliveFeather.

ModernShuntSignal

This (left) is a typicalmodern shunt signal, used to allowmovementsinto and out of a siding. It has three lights with red andwhiteindications.Thesignalcanbeseenatgroundlevelorattachedtoasignalpostbelowanormalstopsignal.Whenmountedbelowastopsignal,theydonotshowan"on"aspect.

TheON indication showsa redandwhite light sideby side.TheOFFindicationshowstwowhitelightsat45degrees.ThenewestoneshavefourlensesandshowtworedlightssidebysideforON.

SomePhotosThe following photos show some of the moremodern colour light signal designs used inBritaintoday.

UK standard 3-Aspect Signal (left) at Sheffield(Midland).Belowtheredaspectisasignwhich,if the signal is showing a proceed indication,illuminates"RA"(RightAway) to tell thedriverstationdutiesarecompleteandhecanstartthetrain.Belowthisisashuntsignal,whichcarriesno red light in this case as the red is alreadyavailableonthemainsignal.Thesignalcarrieda white identification plate and, nearer theground,asignalposttelephone.Photo:Author.

3-Aspect Signal with Theatre Type RouteIndicator.Photo:Author.



Sources:"Two Centuries of Railway Signalling", Kichenside andWilliams, 1998, Oxford PublishingCompany,UK;

"ModernSignallingHandbook",StanleyHall,1996,IanAllanLtd,SheppertonUK;

"SignallingintheAgeofSteam",MichaelAVanns,1995,IanAllanLtd,SheppertonUK.

Documents

British Signalling – What the driver sees v2 · PDF fileOne of a series of papers originally published as pages on RTWP and updated for RTW. Background Paper No.1 Basic Railway Signalling