Circuitron
DT-3 Single-Direction Grade Crossing Detection Unit

Walthers Part # 800-5203, p. 288 Walthers 2012 HO Scale Reference
A scale, $29.95, currently in stock at Walthers

 
Specifically deigned for use at grade crossings, using Opto-Sensors to detect the train. Output controls flasher, gates, bells, etc.
 
Single-direction unit for crossing where bi-directional detection is unnecessary, such as a mainline with trains traveling in only on direction. Operates with any train length. Completely independent of track power. Use in any scale. Requires 10-18V AC or DC input.