Specifically deigned for use at grade crossings, using Opto-Sensors to detect the train. Output controls flasher, gates, bells, etc. Full logic system duplicates prototype grade crossing action. Does not require sensor stay covered to activate. Integrated memory keeps output ′on′ until last car clears the crossing. Activates whenever a train approaches from either direction, If trains stop short then backs away, until will detect that and turn output ′off.′ Independent of train length will detect a short train even if it falls between Opto-Sensors. Includes four Opto-Sensors. 3 x 4′ printed circuit board. 10-18V AC or DC input.