Diesel is a black class 08 diesel engine.
He is a devious engine, who likes to see others in mishaps and likes to cause trouble. However when his brothers were getting scrapped, he and Paxton were bought by Sir Topham Hatt. He is now a bit nicer and doesn't play tricks like he use to. He also apologized to Duck from when they first met and brought Chocolate from Barrow-in-Furness to the works to get repaired.