detector bar

from The Century Dictionary.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• (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.