The Diesel is a Diesel.
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the engines with goods and express work during Stepney's visit, but only insulted the other engines by saying they were out-of-date.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, of course, 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 farewelling Stepney, the Diesel had snuck out, leaving behind "a rather nasty smell, and a battered bowler hat".