Circuitron
DT-4 Rolling Stock Detection Unit

Walthers Part # 800-5204, p. 288 Walthers 2012 HO Scale Reference
A scale, $46.95, currently in stock at Walthers
Ideal for spotting a hidden train or piece of rolling stock. Four
independent detection units combined onto one circuit board, can be used
separately or together to control external devices. Supplied with four
Opto-Sensors that mount between rails to detect movement. When train covers
a sensor, the unit activates any connected DC accessory, such as an
indicator light on a control panel. Independent sensitivity controls are
provided for adjusting each Opto-Sensor to lighting conditions.
Constructed on a 3 x 3" printed circuit board. Requires 10-18V DC.
Independent of track power. Compatible with radio or carrier control
systems.