▁feet ▁( 3 6 5 . 7 5 ▁m ) ▁before ▁a ▁station ▁platform . ▁The ▁" TB " - Sign ▁is ▁posted ▁to ▁indicate ▁to ▁the ▁operator ▁that ▁an ▁eight - car ▁train ▁( the ▁longest ▁used ▁in ▁the ▁system ) ▁is ▁clear ▁of ▁an ▁inter lock ing ▁(" turn back ") ▁when ▁the ▁train ▁front ▁is ▁flush ▁with ▁the ▁sign , ▁and ▁that ▁the ▁train ▁can ▁turn ▁back ▁in ▁the ▁direction ▁from ▁which ▁it ▁came ▁on ▁the ▁opposite ▁track . ▁ ▁Train ▁length ▁▁ ▁When ▁it ▁first ▁opened , ▁Metro ▁ran ▁a ▁mix ▁of ▁four - ▁and ▁six - car ▁trains ; ▁however , ▁there ▁have ▁not ▁been ▁any ▁four - car ▁trains ▁used ▁in ▁the ▁system ▁( except ▁for ▁the ▁money ▁train ) ▁since ▁the ▁mid - 2 0 0 0 s . ▁The ▁most ▁common ▁train ▁length ▁is ▁eight ▁cars ▁due ▁to ▁the ▁fact ▁that ▁the ▁ 7 0 0 0 ▁series ▁rail c ars ▁can ▁only ▁be ▁operated ▁this ▁way . ▁During ▁peak ▁hours , ▁a ▁mix ▁of ▁six - ▁and ▁eight - car ▁train sets ▁are ▁used , ▁while ▁during ▁off - pe ak ▁and ▁week end ▁hours ▁almost ▁all ▁trains ▁are ▁only ▁six ▁cars ▁in ▁length . ▁In ▁ 2 0 0 8 , ▁Metro ▁began ▁planning ▁to ▁ac quire ▁sufficient ▁quantities ▁of ▁the ▁new ▁ 7 0 0 0 - Series ▁cars ▁to ▁run ▁ 1 0 0 % ▁eight - car ▁trains . ▁Metro ▁also ▁tested ▁eight - car ▁trains ▁on ▁the ▁Y ellow