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.