The Diesel was built at the Vulcan Foundry in Newton-le-Meadows, Lancashire, England in the early 1960s. The Diesel was sent to Sodor in 1963 to assist the engines with goods and express work during Stepney's visit, but only insulted the other engines by saying they were out-of-date. 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 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.
- The Diesel's television series number belonged to a real Class 40. The real D261 was built in 1960 and withdrawn in 1983.
- According to the description on Thomas and the Special Letter and other stories, the Diesel was brought to Sodor to take over Daisy's work while she was ill.
- ERTL (discontinued)
- Wooden Railway (discontinued)
- Hornby (with a Co-Co wheel arrangement; discontinued)
- Brio (discontinued)
- Bandai (several versions; discontinued)
- Nakayoshi (discontinued)
- Take-n-Play (labeled as Class 40)
|98462 and 87546 • The Workshop Engines • The Foreign Engines • Diesel • The Foreign Engine • The Derailed Engine • Jinty and Pug • The Diesel • The Scrapyard Engines • 199 • The Passing Diesel • Other Railway Diesels • King James I • Old Stuck-Up • Alfred and Judy • Toby's Brothers • 8783 • The Old Engine • The Diesel Shunter • 31120 • 10751 • Wilbert • Sixteen • Sixteen's Friends • Albert • Bahamas • D5705 • Lemberg • Pretty Polly • The Dukedogs • British Railways|
|Lady • Spencer • Hiro • Hiro's Friends • Connor • Caitlin • Stephen's Friends • The Mainland Engines|
|Silver Jubilee • Helena|
|Spencer's Special Coaches • Connor's Coaches • Caitlin's Coaches|
|The Flying Thistle|