▁Metro ▁was ▁designed ▁to ▁use ▁an ▁Autom atic ▁Train ▁Control ▁( AT C ) ▁system , ▁which ▁compr ises ▁three ▁sub - system s . ▁Autom atic ▁Train ▁Prote ction ▁( AT P ) ▁protect s ▁trains ▁by ▁separ ating ▁them ▁so ▁as ▁to ▁avoid ▁collision . ▁Autom atic ▁Train ▁Super vision ▁( AT S ) ▁routes ▁trains ▁and ▁ass ists ▁in ▁maintain ing ▁ad her ence ▁to ▁schedule . ▁T ogether , ▁these ▁two ▁sub - system s ▁provide ▁input ▁to ▁the ▁train ' s ▁cab ▁signals ▁and ▁to ▁a ▁third ▁sub - system , ▁Autom atic ▁Train ▁Operation ▁( AT O ), ▁which ▁can ▁control ▁the ▁trains ▁directly . ▁Since ▁a ▁train ▁collision ▁in ▁ 2 0 0 9 ▁caused ▁by ▁an ▁A TC ▁mal function , ▁Metro ▁did ▁not ▁use ▁the ▁A TC ▁system ▁following ▁the ▁accident ▁until ▁it ▁was ▁tempor arily ▁re - int rodu ced ▁on ▁the ▁Red ▁Line ▁in ▁September ▁ 2 0 1 4 ▁as ▁a ▁test ▁program . ▁ ▁The ▁Autom atic ▁Train ▁Prote ction ▁sub - system ▁uses ▁c oded ▁track ▁circuit ▁technology ▁originally ▁supplied ▁by ▁Ro chester , ▁New ▁York - based ▁General ▁Railway ▁Sign al ▁when ▁the ▁line ▁was ▁constructed ▁in ▁the ▁ 1 9 7 0 s . ▁It ▁is ▁a ▁life - crit ical ▁system ▁that ▁provides ▁a ▁continuous ▁stream ▁of ▁information ▁to ▁the ▁train ▁regarding ▁the ▁maximum ▁safe ▁speed ▁via ▁the ▁running ▁rails . ▁Speed ▁commands ▁are ▁designed ▁to ▁ensure ▁trains ▁maintain ▁a ▁safe ▁stopping ▁distance ▁from ▁any ▁obst