Diesel was the first diesel engine to arrive on Sodor. A statue of him is currently in Maron Park.
Diesel was purchased in 1935 to be the new Knapford Station pilot. As he often fetches the express coaches he often dreams of pulling the Sodor Express, pulling many coaches with passengers singing his praises.
Diesel was very uninterested when Thomas gloated to him about being chosen for the billboard, citing Diesel wasn't chosen because he was a diesel. When Sir Topham Hatt needed an engine for a special assignment, the only available engine was diesel. Once Thomas arrived however, Diesel was quickly sent to find another engine to pull Annie and Clarabel.
Before being sent on to the Scrapyards to collect a discarded part, Diesel breaks down unexpectedly. Thomas is sent in his place, and he tells Diesel off for taking him away from his usual passenger run. After he is repaired, he follows Thomas to the Scrapyards, finding him trapped by 'Arry and Bert. He leaves quietly to find the Fat Controller, bringing him to the site to address the situation. After being rescued, Thomas apologizes for his unkind exchanges with Diesel in the past.
A few days later, Diesel is invited by Thomas to visit Tidmouth Sheds. While he does not express it, Diesel is very grateful. That night, he overhears the other engines celebrating about the breakdown of the Duke and Duchess' diesel. He discretely leaves after he hears this.
In the aftermath of the Sodor Dieselworks' fire, Diesel misunderstood Percy's joke, and thought he had something to do with Derek's death. That night he arrived at Tidmouth Sheds and furiously confronted the steam engines.
When Diesel 10, Splatter, Dodge, Paxton, D199, Dennis, and Andy came to work on the North Western Railway, Andy convinced the other diesels to form an anti-steam league. After Toby was blown up, Den told him it was Simon Johnson who killed Derek. Diesel escaped the Sodor Steamworks, and joined the steam engines. Some of the engines did not trust him because he was involved in Edward's killing. Shortly after the engines relocated to Transfer Yards, and Gordon's death, James snapped at Diesel, saying that he was responsible for Edward's death and that he was sick of him and his kind coming on railways and stealing his work, before shouting out that he hated him, and told him to leave. however, he saw Andy and Dodge and overheard their plan of using Dodge as a suicide bomber to blow up the remaining engines. Diesel knew he needed to save the steam engines, so he charged into Dodge and Andy, having his vision of him living his dream of pulling the express one last time before crashing into Dodge and detonating the bomb and all three were blown to smithereens. His remains, along with Lady's, Gordon's, Edward's and possibly Andy and Dodge's, were used to make a statue of Diesel honoring the four engines and those who had loss their lives during the conflict.
The model of the deceased body of Diesel was a TWR Chinese knock off, with it's face cut off and heavily weathered.
In Authority, two models of Diesel were used, the 2011 Learning Curve Item, and the 2013 Mattel version during the visions of him pulling the express.
- Season 1: Sodor Day, Tensions Emerge, False Retaliation, Justified Retaliation, Revolutionary, Early Days
- Season 2: Discipline, Authority