Diesel is a devious diesel shunter, originating from The Other Railway.
When Diesel came on trial to the North Western Railway, Duck was asked to show him around. Duck was unimpressed by Diesel's claims of being "revolutionary" and left him to his own devices. As a result, Diesel was made into a laughing stock by the Troublesome Trucks for trying to pull some rusty old trucks and failing spectacularly, and Diesel blamed Duck for not helping him. Diesel swore a vendetta, and spread rumours that Duck had given the big engines rude nicknames. Duck was sent to Wellsworth pending an inquiry into the matter, but Diesel got too big for his buffers and tried telling lies about Henry. He was found out and sent home in disgrace.
Several years later, Diesel was brought back to help on Thomas' Branch Line whilst Percy was being repaired. Diesel tried to win Thomas and Toby over, without success, and after he destroyed several trucks he was to be sent away. However, on his day of departure, Annie and Clarabel slipped on oily rails left by Daisy and came off at some points near Dryaw. Diesel, realising that Thomas was blocking his only way home, went to the rescue. Clarabel put in a good word for Diesel, and Diesel, although he returned to the Other Railway, is always welcome on Sodor.
Diesel returned when Duck and Percy needed help at Brendam. Diesel proved to be as bad as ever and was sent away for good after shunting some trucks of china clay into the sea, but he mysteriously snuck back to give Mavis advice and taunt Gordon about the triumph of Diesel power on the mainland. It is possible that these events happened during Diesel's stay or that Diesel returned at some point with goods from the Mainland.
When Henry was derailed by some trucks, Diesel was brought back as a last resort to do his goods work, but his attempt to prove his worth resulted in him falling in a barge, and he was sent back to the mainland yet again.
Later on, Diesel returned yet again, this time to assist Fergus at the Cement Works. Diesel became jealous of Fergus' reputation as the "pride of the Cement Works" and concocted a scheme to make Fergus run away. Diesel was partially successful - he managed to make Fergus abandon his post for a time - but was ultimately foiled. Amazingly, Diesel was not sent back this time round and remained on the North Western Railway where he was stationed to work at the Sodor Ironworks.
Despite remaining tricky and devious, Diesel learned how useful steam engines can be when Thomas brought fresh diesel fuel for him and the other diesel engines and James pushed him back to The Works when he broke down.