The Diesel was built at the Vulcan Foundry in Newton-le-Meadows, Lancashire, England. The Diesel was sent to Sodor in 1962 to assist the engines with goods and express work during Stepney's visit, but he only insulted the other engines by saying they were out-of-date, and bragged about his modernity. He got his comeuppance when an inspector's bowler hat jammed in his air intake, leaving Duck and Stepney to take the Express for him.
While everyone was fare-welling Stepney, the Diesel crept away, leaving behind "a rather nasty smell of bad manners and a battered bowler hat".
The Diesel is snobbish and rude, believing that diesel engines should take over from steam engines. Despite this, in the magazine stories, there are some occasions in which the Diesel helps the steam engines.
The Diesel is based on a BR Class 40 1Co-Co1. Another member of this class is Old Stuck-Up.
The Diesel is painted in British Railway's two-tone green livery with a black roof and yellow warning panels.
- Michio Nakao (Japan)
- The Diesel's television series number belonged to a real Class 40. The real D261 entered service on the 26th of February, 1960 and withdrawn from service in 1983.
- The Diesel's Railway Series livery is inaccurate as none of the Class 40 diesels received British Railways two-tone green livery.
- According to the description on the Australian/New Zealand VHS Thomas and the Special Letter and other stories, the Diesel was brought to Sodor to take over Daisy's work while she was ill.
- The Diesel was the final Railway Series engine to be introduced to the television series until Glynn in The Adventure Begins.
- His Hornby model depicts him as a Class 37 diesel.
- ERTL (discontinued)
- Wooden Railway (discontinued)
- Hornby (Re-used Class 37 Diesel tooling; labelled as Diesel; discontinued)
- Brio (discontinued)
- Thomas Engine Collection Series (discontinued)
- Nakayoshi (discontinued)
- Take-n-Play (labelled as Class 40)
- Trading Cards (discontinued)
- De Agostini (discontinued)