题目内容：
London’sFirstLightRailSystemTheDocklandsLightRailway（DLR）tookjustthreeyearstobuildatacostof£77million.ItisLondon’sfirstLightRailSystem,butitsroutefollowsthatofanumberofolderlines,whichcarriedthenineteenthcenturyrailwaysthroughthecroweddistrictsoftheEastEnd.
ThesectionofthelinefromtheTowerGatewayStationtoPoplarfollowsthelineofoneofLondon’searliestrailways,theLondon&Blackwell（1840）,acable-drawnrailway（laterconvertedtosteam）whichcarriedpassengerstosteamshipsatBlackwellPier,andprovidedtransportforthemessengersandclerkswhowentbackwardsandforwardsbetweenthedocks（码头）andthecityeveryday.
FromPoplartoIslandGardens,anewlinecrosseshighabovethedockwaters,andthenjoinstheoldtrackoftheMillwallExtensionRailway,builttoservicetheMillwallDocks（1868）andtoprovidetransportforworkersinthelocalfactories.Thislinewashorse-drawnforpartofitsroute,untilthe1880s.
ThePolartoStratfordsectionoftheDLRroutewasfirstdevelopedbytheNorthLondonRailway,builtinthe1850stolinktheWestandEastIndiaDockswiththemanufacturingdistrictsoftheMidlandsandNorthofEngland.Thereweremajorrailwayworksandsidings（岔线）atBowuntilrecently.
Thetrainsareautomaticallycontrolledfromacentralcomputer,whichdealswithallsignalingandothersafetyfactors,aswellasadjustingspeedstokeepwithinthetimetable;onboardeachvehicle,TrainCaptains,whoarealsofullyqualifieddrivers,areequippedwithtwo-
参考答案：
答案解析：