Diesel 10 is a large brown diesel with a claw named Pinchy.
Diesel 10 worked on British Railways before coming to Sodor a long time ago. At the time he was allied with P.T. Boomer and the two chased Lady, making her crash and forced her caretaker to hide her. Diesel 10 was then forced to leave Sodor by Sir Topham Hatt.
Hatt reluctantly brought Diesel 10 back while he was on holiday to help out. However, he brought his two henchmen, Splatter and Dodge, to find and destroy the Magic Buffers as well as the Magic Railroad. After knocking down scaffolding and trying to scrap James, Diesel 10 discovered Lady and Thomas and chased them across the island until they reached a collapsing viaduct. Diesel 10 fell off of it and into a barge and was separated from Boomer. After Sir Topham Hatt returned, he gave Diesel 10 a second chance, and the latter began working in the scrapyards. He later helped to build the Sodor Airport, but briefly reverted to his evil ways when he took over the Sodor Steamworks. After the Sodor Dieselworks was fixed, he went back to being neutral.
Diesel 10 was captured during the diesel invasion and later taken back to the Mainland. He was cloned, but he managed to escape and go into hiding until the war ended.
Diesel 10 and BoCo were resting at the Kildane Scrap and Ironworks when Andy arrived. Even though he knew Andy from the invasion, Diesel 10 stayed silent then, as well as during the meeting held by Professor’s Gang moments late. Although he never sided with them, he didn’t try to stop them either.
A few days later, Diesel 10 passed through Wellsworth Station with a goods train. Jay was a part of the train and he broke the coupling, leaving half of the train on the mainline and the other half with Diesel 10.
The next day, Lady tried to recruit BoCo to stop Andy, but the latter declined and Diesel 10 arrived, declaring he would take his place. Although reluctant at first, Lady teamed up with him, but the two became separated when she went to look for 089. Diesel 10 returned to the Ironworks and met Andy in the smelting shed. Andy revealed his true intentions and a fight broke out as Diesel 10 claimed he would avenge all of the diesels that suffered because of D199 and Andy. However, the latter derailed him and managed to escape, leaving Diesel 10 alone in the burning room as the door closed behind him.
Diesel 10 will play a major role in Season 4.
Diesel 10 used to be a vicious villian bent on the destruction of steam engines, but now he's an old, gruff, utility engine. He usually can be found collecting scrap and sometimes pulling goods trains, but he finds these tasks boring. He hides his emotions under a gruff exterior and usually keeps to himself and only talking to BoCo. Deep down, he misses the other diesels and wants to avenge their deaths. He also would like to be on better terms with Lady, but he keeps that to himself.
- in Quest 89, Diesel 10 is a neutral character, unlike most TWR series, where he is an antagonist.