detector bar

Century Dictionary and Cyclopedia

1. n. A secondary rail or bar placed beside the rails of a switch and intended to prevent its movement while a train is passing over it. The bar is pivoted to the switch in such a manner that when the switch is to be moved it rises above the level of the top of the switch-rail and then returns to its normal position. In rising it strikes the car-wheels, detects their presence, warns the switchman, and also prevents the movement of the switch until the last wheel of the train has passed. See switch.

GNU Webster's 1913

1. (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.