▁boxes ▁to ▁enable ▁trains ▁to ▁run ▁close ▁together . ▁As ▁a ▁result , ▁it ▁was ▁decided ▁to ▁adopt ▁power ▁sign alling ▁under ▁the ▁Autom atic ▁Block ▁System ▁( AB S ) ▁of ▁saf ew ork ing , ▁where ▁the ▁presence ▁of ▁trains ▁automatically ▁control ▁the ▁signals ▁after ▁them , ▁providing ▁a ▁safe ▁distance ▁between ▁trains . ▁Int rodu ced ▁from ▁ 1 9 1 5 , ▁the ▁system ▁was ▁based ▁on ▁American ▁speed ▁sign alling ▁practice ▁with ▁G RS 2 A ▁upper ▁quadr ant ▁mechanical ▁signals ▁with ▁two ▁arms ▁able ▁to ▁indicate ▁up ▁to ▁ 5 ▁different ▁speed ▁aspects ▁to ▁train ▁drivers . ▁These ▁signals ▁were ▁later ▁replaced ▁by ▁colour ▁light ▁signals ▁which ▁are ▁the ▁standard ▁today , ▁but ▁the ▁old ▁mechanical ▁style ▁remained ▁until ▁ 2 0 0 1 . ▁ ▁A ▁variant ▁of ▁the ▁Autom atic ▁Block ▁System , ▁Autom atic ▁and ▁Track ▁Control ▁( AT C ) ▁has ▁since ▁been ▁introduced , ▁which ▁provides ▁the ▁same ▁benefits ▁as ▁A BS ▁on ▁single ▁lines ▁of ▁track , ▁while ▁still ▁ens uring ▁only ▁one ▁train ▁in ▁a ▁section ▁at ▁a ▁time . ▁Central ised ▁Tra ffic ▁Control ▁was ▁also ▁introduced ▁in ▁the ▁ 1 9 6 0 s ▁on ▁the ▁new ▁standard ▁gauge ▁line ▁to ▁Al bury , ▁and ▁then ▁on ▁the ▁main ▁inter state ▁line ▁to ▁Ad ela ide , ▁allowing ▁trains ▁to ▁be ▁directed ▁from ▁a ▁distance . ▁ ▁Today ▁little ▁mechanical ▁sign alling ▁remains , ▁with ▁local ▁signal ▁boxes ▁cont rolling ▁signals ▁abol ished ▁from ▁many ▁areas ▁as ▁part