How does automated railroad crossing gate detect an incoming train?
Lengthy article with photos at the link given above: EXCERPT This circuit provides realistic operation of the flashing signals at level railway crossings. The circuit uses six visible/infrared light phototransistors to control the operation of the flashers and can supply up to 100 milliamps for LED flashers or small incandescent lights. The protected section of track can be as long or as short as desired and does not have to be the same length on both sides of the crossing. The circuit works in both directions and can be connected to other crossing circuits to provide fully automatic protection for multiple track crossings. The circuit is designed to be easy to build and install. Widely available and inexpensive components are used in its construction. Noe of the parts are static sensitive. The output of the flasher circuit can be used to operate crossing gate motors and bell circuits on this site and from other suppliers .
