Why do flashing-light signals or gate arms sometimes activate when no train is in sight? Why do these warning devices occasionally stay on well after the train has passed through the crossing?
The crossing warning devices have a fail-safe electrical relay feature. When the fail-safe feature detects any electrical change in the approach circuit, the warning devices are activated until the circuit is restored to its original status. This should occur when a train first enters the approach circuit to a crossing and alters that circuit's electrical status. However, sometimes there may be a following train or a rail car sitting on the circuit, or the circuit's electrical status was not properly restored after the passage of a train, or the railroad may be testing the circuit for safety.
The length of the circuit is designed so that motor vehicles can clear the tracks before the train reaches the crossing. A minimum of 20 seconds warning must be provided to motorists before a train enters the crossing.