Diesel Becomes a Coach is a magazine story.
Diesel is sent to to the end of Thomas' Branch Line to pick up trucks. At the station, he speaks to Annie and Clarabel. Amongst other things, he asks them why they're so fond of a silly tank engine like Thomas, but the coaches refuse to say anything bad about their friend. This makes Diesel so cross that he forgets to fill up on oil before leaving the station and en route to his destination, Diesel shudders to a stop.
It's not long before Thomas puffs by with his train and kindly offers to pull Diesel back to the station. Soon Diesel is coupled behind Clarabel and all the passengers laugh at Diesel because he looks like a spare coach! Diesel, embarrassed, decides to say nothing.