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)
Other mistake: When Thomas is uncoupled from Annie and Clarabell he has Percy's face.
Answer: Breakdown trains help the engines back on track.