Snow Engine - S7-E14
Other mistake: When Oliver passes the level crossing before he crashes, there is no one in his cab.
Other mistake: When the narrator says that Duncan ran out of coal in the bridge, there is coal in his bunker.
Other mistake: The episode's title in the original PBS Broadcast version is "The Old Iron Bridge" but the bridge was made of wood.
Answer: Butch the breakdown truck must have pulled Gordon's tender all the way up the rails so the breakdown train can reach it and also Butch helped to get Gordon close to the winch.