Character mistake: Throughout the episode, a "Bufferbeam" is referred to as "a buffer." Also, at the beginning, the narrator says that the diesels are shunting coal trucks when they are really shunting flatbeds.
Revealing mistake: In a shot of Gordon arriving at Brendam Docks, only half of his tender is visible.
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.