The crossing protection circuits are either predictors (GCP) which basicly give a constant warning time based on the speed of the train. The train speed is predicted by electrical current flowing down one rail to a certain point where a wire goes from one rail to the other rail and the current flows back to the crossing. As the train enters the curcuit, the electrical current increases as the train approaches the crossing. The computer in the crossing house determines the train speed and activates the crossing. If the train stops in the curcuit, the voltage stabilizes and the gates go up.
This is a very shot explination of what happens.