The Missing Coach is an episode from season 2 of Thomas the Tank Engine.
In the episode, Douglas is stressed about being sent away, & forgets to shunt Thomas's special coach into its siding. The coach is a coach that comes off Gordon's express, & is taken down Thomas's branch line. When Thomas can't find it, he goes to get the Fat Controller. Donald & Douglas swap tenders so The Fat Controller doesn't know who is who. But in the end he finds out & scolds Douglas.
References
The Missing Coach Wikipedia(Text) CC BY-SA