A: No. Here’s why: Crossing circuits have set approaches based off of warning time required and maximum speed of the trains. The modern day crossing processors — mini computers inside a crossing gate bungalow (the metal shed near the tracks) — can run in either prediction mode that provides a constant warning time to crossing for various train speeds or motion detection.
The processor, in either case, detects the shunt/short of the trains leading axles and either predicts when this shunt will arrive at the crossing and activate the crossing at programmed warning time in case of predictor, or just start the crossing in the motion detector mode.
Crossing processors today pass a low voltage current along both rails. The metal wheels and axles of a train complete the circuit — shunt — and the location and speed of the shunt where trains complete the circuit is what the crossing processors are measuring. Signal maintainers can also simulate shunt the track with tools designed to complete the circuit.
With both mode types (predictor/motion detector), once a train stops approaching, the processor detects the stop and will allow the crossing to time out and gates to pick up. Some programming features allow for a motion restart within so many feet of the crossing. The crossing island, which is between the circuit’s transmit and receiver bootleg wires, and extends over the road, is an absolute crossing start and any train or shunt with in it will keep a crossing active.
Typically, a crossing predictor or motion detector is tuned for a value of “100%” of the approach. The unit can detect then where this shunt stops with the approach based off the preset value. Once the approach is completely occupied by the train, it is the island that keeps the crossing active. — A class I railroad signal technician
UPDATED: Oct. 22, 2019, 3:20 p.m. Central time.