How does a railroad crossing work?

Answer 1

There are three main circuits that a railroad crossing uses. There's the two approach circuits on either side, and the island circuit which is the actual crossing itself. Some signal circuits are equipped with timers that will make sure the train has absolutely stopped in the approach circuit and not the island circuit, and time out the signals.

Crossing gates work by a simple relay block in the relay case (where the electrical components are.) that have a timer in them and send a signal for the gate mechanism to bring the gates down and close the road. When the train has left the island circuit and cleared the approach block it came from, the gates will rise and the road will reopen. Crossing bells work the same way with a signal being sent to them.

* Added - The track circuits that activate the crossing systems when a train approaches do not rely on the train "completing" a circuit, as this type of circuit is illegal in the US and many countries. The train either "shunts" the circuit or in the case of electronic circuits, the electronics sense the train, which can include sensing its speed.