Railway Technical Websiterailway-technical.com/archive/track-basics-v2.pdf · 2018-12-28 · Railway Technical Website Background Paper No. 2 Track Basics by Piers Connor1 Introduction

Railway Technical Website BackgroundPaperNo.2

TrackBasicsby

PiersConnor1

IntroductionTrack is thebaseuponwhich the railway runs. To give a train a good ride, the trackalignment must be set to within a millimeter of the design. Track design andconstruction is part of a complex andmulti-disciplinary engineering science involvingearthworks, steelwork, timber and suspension systems - the infrastructure of therailway. Many different systems exist throughout the world and there are manyvariations in their performance and maintenance. This page looks at the basics ofinfrastructure and trackdesignand constructionwithdrawings, photos andexamplesfromaroundtheworld.SomeinformationwascontributedbyDanMcNaughton,SimonLoweandMikeBrotzman.

BackgroundThetrackisafundamentalpartoftherailwayinfrastructureandrepresentstheprimarydistinctionbetweenthisformoflandtransportationandallothersinthatitprovidesafixedguidancesystem.Thetrackisthesteeringbaseforthetrainandhasevolvedfroman ancient design of vehicle guidance with origins dating, some historians havesuggested,fromtheSumeriancultureof2000BC.Themodernrailwayversionisbasedon the steel wheel running on a steel rail. Other forms of guided vehicle technologyexist; rubber-tyred trains, magnetic levitation and guided busways, for example, butthesearenotdealtwithhere.There isagooddescriptionof theFrench-basedrubbertyredtraintechnologyavailableatTheRubberTyredMetrowebsite.

BasicConstructionTrackisthemostobviouspartofarailwayroutebutthereisasub-structuresupportingthetrackwhichisequallyasimportantinensuringasafeandcomfortablerideforthetrainanditspassengersorfreight.Theinfrastructurediagramhereshowstheprincipalpartsofanelectrified,double-trackline.

Thetotalwidthacrossthetwo-trackalignmentwillbeabout15m(50ft)foramodernformation. The "cess" shown each side of the alignment is the area available for awalkwayorrefugeforstaffworkingonthetrack.

TheSub-StructureThispartoftheroadconsistsofthreemainelements;theformation,thesub-ballastandtheballast.Theformationisthegrounduponwhichthetrackwillbelaid.Itcanbethenaturalgroundlevelor"grade"oritcanbeanembankmentorcutting.It isimportantthattheformationismadeoftherightmaterialsandisproperlycompactedtocarrytheloadsofpassingtrains.

Theformationunderthetrackhasa"camber"ratherlikethatseenonaroadway.Thisis toensureeaseofwaterrun-off to thedrainsprovidedoneachsideof the line.The

1PRCRailConsultingLtd.

OneofaseriesofpapersontechnicalissuespublishedbytheRTWfromtimetotime.ThispaperisbasedonapageontheoriginalRailwayTechnicalWebPagessite.

BackgroundPaper TrackBasics

RailwayTechnicalWebsite Page2 Updated10thMay2017

track itself issupportedon"ballast",madeupofstones -usuallygraniteor, in theUS,basalt-belowwhichisalayerofsand,whichseparatesitfromtheformation.Forneworrenewedformations,thesandisnormallylaidoversomesortofgeotechnicalscreenormeshtoseparateitfromthefoundationmaterialbelow.Inthepast,asphaltorplasticsheetinghasbeenusedtopreventwaterseepage.

Catenarymasts(ifthelineiselectrifiedontheoverheadsystem)arelocatedoutsidethedrainsand,beyondthem,thereisawalkwayarea.Thismayjustbeaclearedpathforstaff to walk safely, avoiding passing trains or, on modernised routes, a properlyconstructed path. Next to this path will be a cable trough. These were originallyconcrete but are nowadays often made of plastic. Cables crossing the track areprotectedbyaplastictube,usuallybrightorangeintheUK.Propercableprotectionisessentialtopreventdamagebyanimals,trackmaintenancetools,weatherandfire.

Usually,theedgeoftherailwaypropertyisoutsidethepathwayorcableruns.Ifthelineisbuiltthroughanarearequiringanembankmentorcutting,theslopeswillbecarefullydesignedtoensurethattheangleofslopewillnottakeanexcessivewidthof landandallowproperdrainagebutwithoutriskinganearthslip.Theslopeangledependsonthetypeofsoilavailable,theexposure,theclimateandthevegetationinthearea.Drainageditchesareoftenaddedalongtheedgesofcuttingsandembankments.IntheUK,fencesarealwaysprovidedalong theboundary lineof therailway toprotect thepublic fromwandering onto the track. Even so, there are a few accidents every year whentrespassers are killed or injured by trains or electric conductor rails.Many countriesaroundtheworlddon'tfencetheirrailways,assumingpeoplewilltreatthemlikeroadsandlookbothwaysbeforecrossing.Theydon't.

BallastBallastisprovidedtogivesupport,loadtransferanddrainagetothetrackandtherebykeepwater away from the rails and sleepers. Ballastmust support theweight of thetrackandtheconsiderablecyclicloadingofpassingtrains.Individualloadsonrailscanbeashighas50tonnes(55USorshorttons)andaround80shorttonsonaheavyhaulfreightline.Ballastismadeupofstonesofgraniteorasimilarmaterialandshouldberough in shape to improve the locking of stones. In this way they will better resistmovement.Ballaststoneswithsmoothedgesdonotworksowell.Ballastwillbelaidtoadepthof9to12 inches(upto300mmonahighspeedtrack).Ballastweighsabout1,600to1,800kg/cu/m.SeealsoBallastedvsNon-BallastedTrackbelow.

TrackTheusual track formconsists of the two steel rails, securedon sleepers (or crossties,shortened to ties, in theUS) so as to keep the rails at the correct distance apart (thegauge) and capable of supporting the weight of trains. There are various types ofsleepersandmethodsofsecuringtherailstothem.Sleepersarenormallyspacedat650mm (25 ins) to 760 mm (30 ins) intervals, depending on the particular railway'sstandardrequirements.

Sleepers(Ties)Traditionally,sleepers(knownastiesintheUS)arewooden.Theycanbesoftwoodorhardwood. Most in the UK are softwood, although London Underground uses ahardwood called Jarrah wood. Sleepers are normally impregnated with preservativeand,undergoodconditions,willlastupto25years.Theyareeasytocutanddrillandusedtobecheapandplentiful.Nowadays,theyarebecomingmoreexpensiveandothertypesofmaterialshaveappeared,notablyconcreteandsteel.



Concrete is the most popularof the new types(left). Concrete sleepers aremuch heavier than woodenones,so theyresistmovementbetter. Theyworkwell undermost conditions but there aresome railways which havefoundthattheydonotperformwell under the loads of heavyhaul freight trains. They offerless flexibility and are allegedto crack more easily underheavy loads with stiffballast. They also have thedisadvantage that they cannotbecuttosizeforturnoutsandspecial trackwork. A concrete

sleeper can weighs up to 320 kg (700 lbs) compared with a wooden sleeper whichweighsabout100kgor225lbs.Thespacingofconcretesleepersisabout25%greaterthanwoodensleepers.

Another type of concretesleeper is shown in thisdrawing (left). This is thetwin-blocksleeper.Thedesignconsistsoftwoconcreteblocksjoinedbyasteelbar.Itis30%lighterthanaregularconcretesleeper, allowing it to bemovedmanually. It ispopularin France (where it is called

Stedef)andforsomelightertrackformslikethoseusedfortramwaysystems.Here isanexampleinSheffield(below).

Thephotoshowstwinblockandwoodensleepersinthesametrack.Thesleepersshownintheabovephotoaresupplementedwithwoodensleepersatthecrossover,becauseitiseasiertocutthetimbertothecorrectsize.Sleepersatcrossoversandturnoutsvaryinsizeaccordingtotheirpositioninthelayout.

Steel sleepers are also now used onmore lightly used roads, but they areregardedassuitableonlywherespeedsare100mi/h(160km/h)orless.

In the US most ties are made of oaksoaked in creosote, cost on averagebetween $22-$29 each. Iit used to beup to $35-$40 per tie in the late80's. They can last up to 20years. Most Class 1 RR's will replacethem after 5-10 years and then sellthem as used. Concrete ties are $42dollars. They are popular in thewestern US and on passenger lines intheeast.Recently,compositetieshave



come on the market. They are made of something like old tires and recycledplastic.Theycanbeusedandspikedlikeregularties,costabout50%lessandsaveontrees.

RailThe standard form of rail usedaroundtheworldisthe"flatbottom"rail. Ithasawidebaseor"foot"andnarrower top or "head". The UKintroduced a type of rail which wasnot used elsewhere - apart from afewUKdesigned railways.Thiswasknown as "Bullhead" rail and isshown in comparison with thestandardtypeinthediagram(left.)

Bullheadrailwasoriginallydesignedwithreuseinmind.Itwasintendedthatitwouldbe turned over when the top had worn but this proved impossible because theundersidealsoworewhere ithadbeensecuredtothesleeper.Bullheadrailhastobemountedinaspecial"chair"madeofcastironandsecuredbya"key"wedgedbetweenthe rail web and the chair. The chairs are secured to the sleepers by "coachscrews".Thearrangementcanbeseeninthefirstphotobelow:

BullheadRails

FlatbottomrailThesecondphotoleftshowsaflatbottomrailclippedtoabaseplate under therail. Flat bottom rails canalso be "spiked" directly tothesleepers.Awideheadednail is driven into thesleeper on each side of therail so that the foot of therail is held by the heads ofthespikes.Longstretchesoftrack were laid in recordtimes across the US in thepioneering days of railroad



developmentusingthismethodofsecuringrailsto"ties".Nowadays,heavierloadsandfastertrainsrequiremoresophisticatedsystems.

Normally, the rail rests on a cast steel plate which is screwed or bolted to thesleeper.Therailisattachedtotheplatebyasystemofclipsorclamps,dependingonthedesign. The olderUK standard designwas an elastic spikewith a sprung, curved topwhichsecurestherail.Thereareanumberofvariationsseenaroundtheworld.Oneofthe most popular is the "Pandrol" clip seen above. A resilient pad will be providedbetween the rail and the base plate and around the securing clip, where required toprovideinsulationforthetrackcircuits,ifinstalled.

The infrastructureowningcompany in theUK(currentlyknownasNetworkRail),hasadoptedUIC60rail(whichweighs60kg/mor125lb/yd)asitsstandardforhighspeedlines. The present standard is equivalent to theUIC 54 rail,whichweighs about 113lbs/ydor54kg/m.Didyouknow,thereare2,400sleepers(ties)inamileoftrack?

IntheUS,railweightvarriesfrom80-90lb/yd(pounds/yard)insmallyardsto100-110lb/ydonlightdutytrackandbetween130and141lbsonheavydutytrack.Railof141lbs is thenewmain linestandard.ThePennsylvaniaRRusedaspecial155 lb/ydrail,whichwas theheaviesteverrolled formainlineoperations.Someof it is still inplacewith8-bolt joints insteadof themoreusual6-bolt joints. It isoveran inch taller thancomparable141poundrail.Highestpricesteelrailcosts$700aton(2000lbs).A133poundrailcosts$46.55peryard(0.92m.).

Older track is jointed. In the UK, about 35% of track is still jointed, although this iscontinuouslyfallingasnewrailisinstalled.Railswerenormallylaidinstandardlengthsboltedtogetherbywhatarecalledfishplates intheUKorsplices intheUS.The jointsallowedsufficientspaceforexpansionastheywereprovidedat60footintervalsintheUKand39footintheUS,allowingthemtobecarriedinastandard40ftflatwagon.Thejointswerealwaysstaggered in theUSwhereas theUKplacedthemsidebyside.TheresultoftheUSstaggeredjointscanbeseeninthecuriousrollingmotionoffreightcarsrunningonpoorlymaintainedtrack.Thereasonforthestaggering is that, intheUS itwasdeterminedthatafter jointedrailhasbeeninplaceforatimeitstartstodropandcreates adepression.Whenawheel falls into thedepression andbegins to climboutagainitexertsaforce.Ifthetwojointswereinparallelthisforcewouldbemuchlargerand the joints might snap. This is not considered a problem in the UK and Europe,probablybecauseofthelighteraxleloads.

Nowadays, rail iswelded into longlengths, which can beup to several hundredmetreslong.Expansionis minimised byinstalling and securingthe rails intension. Provided thetension is adjusted tothe correct level,equivalenttoasuitablerail temperature level,expansion joints arenot normallyneeded. Special jointsto allow rail

adjustmentareprovidedatsuitablelocationsasshowninthephotoabove:



Adjustment switches are also provided to protect turnouts and at locations where achangeintheraildesignorsizeoccurs.

Rail tends tocreep in themaindirectionof travel so "railanchors" ("anti-creepers" intheUS)areinstalledatintervalsalongthetrack.Theyarefittedundertherailagainstabaseplatetoactasastopagainstmovement.

RailWeldingModerntrackworkuseslongweldedrail lengthstoprovideabetterride,reducewear,reducedamagetotrainsandeliminatethenoiseassociatedwithrailjoints.Railweldingisacomplexart(orscience)dependingonhowyoufeelabout it.Therearetwomaintypesofweldingusedforrails:ThermitweldingandFlashButtwelding.

GaugeThestandard trackgauge - thedistancebetween the tworails - is4 ft.8½ inor1435mm. but many other gauges, wider and narrower than this, are in use around theworld. Gauge is often intentionally widened slightly on curved track. There is someadditionalinformationonTrackGaugesatthePacificSouthwestRailroadMuseumsite.

ModernTrackFormsTherearenowarangeofmoderntrackformsusingaconcretebase.Theyaregenerallyused in special locations such as tunnels or bridgeswhere a rigid base is required toensure track stability in relation to the surrounding structures. This type of track,usually called "slab track" or "non-ballasted" track, often appears as shown in thediagrambelow:

The earth mat is asteel mesh screenprovided onelectrified railwaysto try to keep strayreturncurrentsfromconnecting toutilities pipes andnearby steelstructures. Earthingmust be strictlycontrolledotherwiseserious and

expensiveproblemswilloccur,mademoreseriousandexpensivebecausetheyinvolveotherpeople'sproperty.

Someslabtracksystemshavethesleepersrestingonrubberorsimilarpadssothattheybecome"floatingslabtrack".Floatingtrackisusedasawayofreducingvibration.HongKong Mass Transit Railway is fond of it, since its lines run through very denselypopulatedareas.

BallastedvsNon-BallastedTrackThebasicargument fordifferenttrackdesignswillbebasedonthebottomline-cost;cost of installation and cost ofmaintenance. There are however, other issues such asenvironment-noise,dustandvibration-orengineeringissuessuchasspace,location,climateandthetypeofserviceintendedforthetrack.

There are a wide variety of track forms and systems incorporating some form ofconcrete base or supportwhich doesn't need ballast. Almost all of these require less



depthofconstructionthanballastedtrack.However, theaccuracyof installationmustbe higher than that needed for ballasted track. Slab track will not be adjusted afterinstallationbutballastcanbepackedtoaligntrackasrequired.

The ability of ballast to allow track realignment is one of its most seriousweaknesses.The lateralmovementcausedbypassing trainsoncurvedtrack isoneofthemajor causes ofmaintenance costs added towhich is the crushing causedby axleweight and damage due to weather and water. Ballast damage leads to tracks"pumping" as a train passes and, eventually, rail or sleeper damagewill occur, to saynothing of the reduced comfort inside the train and the additional wear on rollingstock. Apart from regular repacking or "tamping", ballast will have to be cleaned orreplacedeveryfewyears.

Another aspect to the ballasted track design, is the dust which is caused duringinstallation and as it wears or gets crushed. It does however, offer a useful sounddeadeningquality.

Fixed track formations using slab track or a concrete base of some sort do not sufferfrom suchproblems.However, the installation of slab track is reported to cost about20%morethanballastedtrack.Tobalancethiscost,themaintenancecostshavebeenquotedasreducedby3to5timesthatofballastedtrackonahighspeedlineinJapan.

Iflowlevelsofuseareforeseen,oriflowcapitalcostisamoreimportantrequirement,ballasted trackwould be the choice. For a heavily used railway, particularly one in astructurallyrestrictedarealikeatunnelorviaduct,non-ballastedtrackmustbethebestoptionongroundsoflowmaintenancecostandreducedspacerequirements.However,caremustbetakenduringdesignandinstallationtoensurethebestoutofthesystem.

StructuresToensurethatthepathrequiredforthepassageoftrainsiskeptclearalongtherouteofa railway, a "structure gauge" is imposed. This has the effect of forming a limit ofbuildinginsidewhichnostructuresmayintrude.Thelimitincludesnotonlythingslikewalls, bridges and columns but also pipes, cables, brackets and signal posts. The"structure gauge" will vary with the curvature of the line and the maximum speedsallowedalongthesectioninquestion.

Althoughthecivilengineerispreventedfromallowinghisstructuretointrudeintothetrainpath,therollingstockengineeralsohaslimitsimposedonthespacehistrainmayoccupy.Thisspaceisreferredtoasthe"kinematicenvelope".Thisareadesignatesthelimits the train canmove laterally andvertically along the route.As for the structuregauge,thekinematicenvelopewillbeaffectedbyspeedandfeaturesoftraindesignsuchasthebogiesuspensionandspecialsystemsitmayhaveliketilting.

GaugingThelineofroutehastobecheckedfromtimetotimetoensurethatthestructuresarenotinterferingwiththegauge.Alineisalwaysgaugedwhenanewtypeofrollingstockis to be introduced. It is important to see that the small variations in track position,platformedge, cableduct location and signal equipmenthasn't been allowed to creepinwardsduringmaintenanceandrenewalprogrammes.

Gauging used to be done by hand locally (and still is from time to time in specialcircumstances) but nowadays, it is mostly done with a special train. The train usedconsistofaspecialcarwithawoodenframebuiltalmosttothegaugelimits.Theedgesoftheframewerefittedwithleadfingerssothat,iftheyhitanythingasthetrainmovedalong,theywouldbendtoindicatethelocationanddepthofintrusion.



Modern gauging trains are fittedwith optical or laser equipment. The optical systemuseslightstospreadbeamsoflightoutfromthetrainasitrunsalongtheline.Suitablymounted cameras record the breaks in the light beams to provide the gauginginformation.Thetraincanrunatupto50mi/h(80km/h)but,ofcourse,therunshavetobedoneatnight.Laserbeamsarealsousedbut,astheyrotateroundthetrainandforma"spiral"of light, themethodsuffers fromgapswhichcanallowintrusionstobemissed.

MonumentsandDatumPlatesAlong the line of route various locations aremarked by a fixed post in the track or aplateonanearbystructuretoindicatethecorrectlevelorpositionofthetrack.Thesearecalledmonumentsordatumplates.Measurementsaretakenfromthesetoconfirmthecorrectpositionofthetrack.

CurvesCurvesinthetrackarealmostascienceontheirown.Carefulcalculationsarerequiredtoensure thatcurvesaredesignedandmaintainedproperlyand that trainspeedsareallowedtoreachareasonablelevelwithoutcausingtoomuchlateralstressonthetrackorinducingaderailment.Therearebothverticalcurvesandhorizontalcurves.Thereisalsoasectionoftrackoneithersideofacurveknownasthetransition,wherethetrackis changing from straight to a curve or from a curve of one radius to one of anotherradius.

CantCantisthenameusedtodescribethecrosslevelangleoftrackonacurve,whichisusedtocompensateforlateralforcesgeneratedbythetrainasitpassesthroughthecurve.Ineffect,thesleepersarelaidatananglesothattheouterrailonthecurveisatahigherlevelthantheinnerrail.IntheUS,itisknownassuperelevation.

Of course, therewillusuallybe trainsofdifferent types,permitted speedsatdifferentlevels,which travel thesamecurve.Also, therewillbeoccasionswhen trains stoponthecurve.Thismeansthatthedegreeofcanthastobefixedatacompromisefiguretoallowthesafetyofstoppedtrainsandthebestspeedsforallthetrainsusingthecurve.

Inpractice, faster trainsareallowedto travel roundthecurveataspeedgreater thantheequilibriumlevelofferedbythecantsetting.Passengerswillthereforefeelalateralacceleration similar to what they would feel if there was no cant and the train wastravelling at a lower speed round the curve. The difference between the equilibriumcantrequiredbythehigherspeedandtheactualcantisknownasthecantdeficiency.

Cant ismeasured either in degrees or in linear dimensions. On standard gauge track(1435mmor 4ft. 8½ins.) 150mmor 6 ins. of cant is equal to 6 degrees. This is thenormalmaximum in theUK. Themaximumamountof cantdeficiency allowed is 110mm(4½ins.).

TurnoutsI have used the word "turnout" to describe the junctions in trackwork where linesdivergeorconvergesoastoavoid"points"(UK)or"switches"(US),bothofwhichtermscan be confusing. In the railway "trade", turnouts are referred to as "switch andcrossingwork".Aturnout(diagrambelow)consistsofanumberofpartsasfollows:



Themovingpartoftheturnoutistheswitch"blade"or"point",oneforeachroute.Thetwoblades are fixed to each other by a tie bar to ensure thatwhenone is against itsstock rail, the other is fully clear andwill provide room for thewheel flange to passthroughcleanly.Eithersideof thecrossingarea,wingandcheckrailsareprovidedtoassisttheguidanceofthewheelsetsthroughthecrossing.

CrossingsThe crossing can be cast orfabricated. Rails are usually madeofsteelwithalargeironcontentbuta little manganese is added tocrossings and some heavily usedrails to increase resistance towear. Below is a photo of anexample of a cast manganesecrossing. A crossing is alsosometimesreferredtoasa"frog".

TypesofTurnoutsThereareanumberofstandardlayoutsortypesofturnouts,asshowninthefollowingdiagrams.





Thesecanbefoundanywherebutthetrendistomakelayoutsassimpleaspossibleinorder to reduce installation and maintenance costs. The more complex layouts areusuallyonlyusedwherespaceislimited.

ExamplesofTurnoutsTheusesofturnoutsarewiderangingandcovermanyvariations.Afewexamplesareofferedbelowtoshowthediversityavailable.

Trap Points are provided atthe end of a siding or loopline to protect the main linefrom a train or vehicleswhich accidentally passbeyond the limits of thesiding. They are normallyunpoweredtrailingpoints,i.e.they allow a train to passsafely through one directionbutwillcausethetrain tobederailed if it passes in thewrongdirection.

Similar points called CatchPointswereoftenprovidedatthelowerendofagradienttoderail runaway vehicles. Inthe photo shown here, thepoints are provided at thelimitofauthorisedshunting.

HighSpeedSwitchHigh Speed trains requirehigh speed turnouts. InJapan, the so-called "bullettrain" or "Shinkansen" hasspecial routes andtrackwork. Turnouts aredesigned for 160 km/h (100mi/h) operation. In theexample shown here, thereare seven point motors tooperate the very long and



heavy switch blade. Similar turnouts are provided for the TGV high speed lines inFrance.

SwitchedCrossingAswitchedcrossing(sometimesreferredtoasaswingnosecrossingormoveablefrog)willnormallybeprovidedforturnoutswithaveryacuteangle.Thecrossingwillhaveapoweredelementwhichwillbesetfortherequiredrouteatthesametimeastheswitchbladeisset.

The blades of aturnout are normallymovedremotelyusingan electricallyoperated pointmachine. Themachine contains thecontacts whichconfirmthepointsaremoved and locked inthe correct positionfor the routeset. Point machinesare normally locatedto one side of thetrack but a newgeneration ofmachines is nowappearing where themechanism is

containedinasleeperfittingbetweentherails.

USSwitchMachineInsomepartsoftheUS.electro-pneumaticpoint machines areused. They arereferred to as switchmotors. The LondonUndergroundalsousede.p. motors. Theyrequire an air main tobe laid alongside thetrack and compressorsto supply theair.Theycan also causeproblems withcondensation due toclimatic changes. Thisphoto also shows aheaterusedtokeepthe

turnoutbladesfreeoficeandsnowduringbadweather.



Sources:Railway Age; Modern Railways; International Railway Journal; Railway GazetteInternational;MassTransit;TrainsMagazine.

Documents

Railway Technical Websiterailway-technical.com/archive/track-basics-v2.pdf · 2018-12-28 · Railway Technical Website Background Paper No. 2 Track Basics by Piers Connor1 Introduction