The Diesel is snobbish and rude, believing that Diesel engines should take over from steam engines.
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the engines with goods and express work during Stepney's visit, but only insulted the other engines by saying they were out-of-date.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 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 is real. The real D261 was built in 1960 and withdrawn in 1983.