|Railway Series Portrait|
|Position||S1, Main yard shunter|
|Age||54-59 (as of 2011)|
Diesel was sent on loan from British Railways to the Island of Sodor in 1957 to help the Fat Controller. Unfortunately, he couldn't handle the Troublesome Trucks and was sent packing after he made offensive nicknames at Henry, Old Square Wheels, James, Rusty Red Scrap Iron, and Gordon, Galloping Sausage, by talking to the trucks and Duck as the prime suspect behind it. He was first seen at the end of Limits, where he arrived at the sheds to replace Donald. The other engines knew this wasn't going be good, and it wasn't. In Tolerance, he didn't get Douglas ready with his passenger coaches by blocking them with trucks early next morning. The same night he used some trucks to block James and Henry out of the sheds but with The Fat Controller sending him packing, he removed the trucks before going home and in a disgraceful moment of truth.
Diesel arrived to do the project of the Arlesdale Railway, but left him derailed by the trucks near a steep cliff. He later worked on Thomas' Branch Line since the two tank engines fell out with each other and sent Diesel off it before he met The Fat Controller again. He rescued Gordon with the Yuletide Express later on and escaped to the North West Railway despite one of his brothers being scrapped in 1967. The locomotive appeared in many episodes including his story in 1957 named Bad Apple and in Worked Out which he took industrial action being overworked since hearing about a similar story from Henry about the big engines going on strike more than thirty years ago.
Diesel spoke to James at both Knapford Harbor and at Knapford Sheds in the episode, Fame & Misfortune. The harbor is also where he spoke to Barry who then started a fight with him over his brothers' withdrawn from service in 1965, while Diesel's brothers were withdrawn in 1967. Iris did speak to him about a long forgotten memory where Diesel made his biggest mistake yet by making nasty nicknames on Henry, Gordon and James while blaming this on Duck as the Troublesome Trucks were laughing loudly in Knapford Yards. In Taking Care of Christmas, Diesel told Iris about her new "back door" plan when the "front door" out of The Container Yard was blocked by a derailed Patrick and his train of shipping containers left out on The Main Line by accident.
In Troublesome Trucks, Duck turns to Diesel for advice on how to find the culprit for who is causing trouble on his branchline. Wanting to this a step further, Diesel wishes to use his manipulation skills on the trucks again, but this time, to stop their tricks. After a bit of hesitation, Duck accepts this help, and Diesel is determined to not only stop this, but also to fully atone his mistakes to Duck.
- Season 1 - Limits and Tolerance
- Season 2 - Little Engines, Old Tricks, Diesels and Dukes and Galloping Sausage
- Season 3 - Search Engines, Bluebells & Batsmen, First Impressions, Gordon's Promise, Shakedown, Escape (cameo), Bad Water (cameo), Suspicions, Henry Sees Red and Christmas Delivery (cameo)
- Season 4 - Medivac (cameo), One Good Deed, Worked Out, Iron Hero (mentioned) and Post & Prejudice (mentioned)
- Barry the Rescue Engine - The Lucky Ones (cameo) and A Troubled Engine
- Season 5 - Doubts (cameo), Expectations (mentioned), Darkest Hour (cameo) and Desperate Times (cameo)
- Season 1 STMY - A Change of Scene (mentioned), Tornado (cameo), Fame & Misfortune, Separation Anxiety (does not speak), Last Laugh and Taking Care of Christmas
- Season 2 STMY - Swan Song (cameo), The Thin Clergyman, Troublesome Trucks, The Devious Way and Endgame
- In The Early Years, Diesel's old Sodor Island 3D made for TRS2006 was used, as it was the best model available. For The Modern Years, WildNorWester created a brand-new model from scratch that is to the correct scale of a Class 08 and has a much friendlier looking face fitting his new personality.
- WildNorWester gave Diesel a new paint scheme of BR Green with Yellow Lining, which is basically the standard NWR diesel scheme carried by all diesels, minus Daisy, BoCo, and Mavis (the latter of which, is privately owned anyway), but decided to keep it as black after vehement criticism. This was originally a private model of Diesel, but after using it in 'Endgame', WildNorWester made the model public.
- Diesel's appearance in season 2 was improvised pretty much on the spot. WildNorWester wanted to see what would happen if Diesel was faced with one of his brothers being cut up for scrap, and how he would develop in the series overtime.