I've always wondered this.

Many times I see a train, say, going 40mph and the crossing gates are automatically activated while the train is some distance down the track. Other times, I see the train pulling up to the crossing and stopping just short of the grade crossing. The gates are activated at around the area the faster train triggered the sensor, but then as this slower train stops, the gates go back up, allowing vehicles to safely use the crossing. Once the train starts rolling, the gates come back down to stop the vehicles.

How is this done? Is it a manual override activated by the crew via remote control?
Thanks,

almo