This circuit illustrates the function of a simple AC-DC relay circuit to operate a railroad-highway grade crossing signal. I included a yellow pilot LED near the approach circuit switch and a red pilot LED near the island circuit switch.
When a train enters the approach circuit from either side, the warning circuit engages. As the train enters the island circuit (short distance of track where crossing is located) the flop relay actuates. The train remains within the approach circuit as it clears the island circuit. The flop relay allows the warning to disengage as the train clears the island circuit. After the train clears the approach circuit, the flop relay is restored.
If a train should ever stop after clearing the island but before restoring the flop relay, the warning will not actuate again until the train reverses direction and enters the island circuit.
If the approach relay should fail or is disabled, then when a train enters the island circuit it will engage the flop relay and the warning circuit will engage.
