The signal is a 124mm (4-7/8") unit.
The sensor is built into the signal track & senses any loco, coach or wagon entering it, the light will turn red, when the last unit passes the sensor it turns yellow, then after 2-3 seconds it will turn back to green.
If you run against the signal it will turn red. No Alteration Is need to any of your stock.
Tthe signal uses "pure" 12 Volt DC so it is recommended to use a 24-844 Signal supply unit, which will power up to 3 signals.
The signal can also be used in a single viaduct layout by mounting it in a KATO # 20-421 Viaduct section.