The Collaborative International Dictionary of English v.0.48:

Detector bar \De*tect"or bar\ (Railroads)
   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.
   [Webster 1913 Suppl.]