Other mistake: When Thomas is uncoupled from Annie and Clarabel, he has Percy's face.
Audio problem: When Sir Topham Hatt goes "Mmmmmmm..." if you listen closely, you'll hear Ringo Starr's lip smacking in the background very faintly. This goof is only visible in the American version of the episode. (00:03:00)
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.