D defs.my
Entry 1 sense Webster, 1913

Detector bar

· De·tect·or bar · IPA
01 A bar, connected with a switch, longer than the distance between any two consecutive wheels of a train (45 to 50 feet), laid inside a rail …
  1. 1.
    A bar, connected with a switch, longer than the distance between any two consecutive wheels of a train (45 to 50 feet), laid inside a rail and operated by the wheels so that the switch cannot be thrown until all the train is past the switch.(Railroads)