Thomas, Percy and the Dragon - S3-E7
Revealing mistake: When Thomas asks Percy about the guard leaving the light on, Thomas' right eye jitters.
Revealing mistake: While the passengers are evacuating the station, the platform moves, a pillar in the background moves, and putty can be seen on the platform (stains of it) and on some passengers' feet.
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.