- "Sir Topham Hatt should scrap you, and get engines like me."
- — The Diesel, Bowled Out, fourth season
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goods and express work toward the end of Stepney's visit, but he only insulted the other engines by saying they were out-of-date, and bragged about his modernity.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, causing him to break down and be as sick as boiler sludge. He sulked in the shed while Duck and Stepney took 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, 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. Old Stuck-Up and D782 are other members of this class. The Diesel's television series number belonged to a real Class 40. The real D261 entered service on the 26th of February 1960 and was withdrawn from service in 1983, being scrapped at Crewe in March the following year. It was renumbered to 40 061 in 1973. Seven Class 40s are preserved, the first built, D200, at the National Railway Museum.
The Diesel is painted in British Railways' two-tone green livery with a black roof and yellow warning panels. In the television series, the Diesel is painted in British Railways' Brunswick green livery, with a thin, lighter green stripe running along his top and yellow warning panels. His number is painted on his cab sides in yellow.
In the Railway Series, his face was yellow, while in the Television Series, it was grey like the other engines.