Circuitron
DT-1 Grade Crossing Detection Unit

Walthers Part # 800-5201, p. 288 Walthers 2012 HO Scale Reference
A scale, $36.95, not currently in stock at Walthers, Expected: Unknown
This product is on-sale today for $30.98

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Specifically deigned for use at grade crossings, using Opto-Sensors to detect the train. Output controls flasher, gates, bells, etc.
 
Detects train when an Opto-Sensor is covered. Senses track polarity and activates only the Opto-Sensors for the direction of travel. Always turns off after the last piece of rolling stock clears the crossing. Includes four Opto-Sensors. Requires an Alternating Flasher Unit to operate flashing lamps. Note: DT-1 cannot detect a short train or loco if it falls between Opto-Sensors. Use in any scale with DC track power. 2 x 3" printed circuit board. 10-18V AC or DC input.