"I've sabtoaged the coal truck, now Connor and his friends will have less of a chance of finding engine 89 before us!"
Diesel was a devious diesel from the Mainland.
Diesel worked on Sodor for many years until the start of the Locomotive Wars where he somehow became a high ranking official in the army. He then returned to Sodor and was tasked with eliminating 089.
When Connor arrived on Sodor, Diesel was on the coastal track with Thomas, Percy and Ivo Hugh. He decided to let them know he was looking for 089 so that they would trust him. After they ran out of coal, he sabotaged a truck that was going to deliver coal to them. He then address his lackeys, Splatter and Dodge, to take out the engines one by one. After failing to catch Ivo Hugh, Diesel himself captured Thomas and transported him to a scrap yard far away with help from Frank. He was later stopped by Ivo Hugh when the scrapyard was blown up by his own bomb. After returning to Sodor and his base, Connor and the other engines invaded. After being defeated, he encountered the group again in a snowy forest. As a final attempt to stop them from finding 089, Diesel tried to blow Connor and his friends up with a fuel tanker and an oil tanker with help from Mr. Railway Man. The two failed and were arrested.
In between seasons, the two managed to escape prison. After escaping from prison, Diesel stumbled upon a type of super fuel that could charge diesels and improve their stats. He brought in a large majority of The Diesel Army to Sodor to help construct a mining colony.
After only creating one mining station, BoCo gained knowledge of Diesel's plan. He informed Ivo Hugh and the latter brought his friends along with him in attempt to stop Diesel. Diesel managed to become charged with the fuel, but he was the only to become charged before the operation was stopped by Connor and his friends, but ultimately by Charlie. The latter pushed Diesel in molten lava where the fuel was being mined from but ended up falling in alongside him.
Diesel was devious and full of surprises, to say the least. In the past he usually played tricks on the engines and spread rumors with the trucks, but in his final weeks, he tried to track down 089. He alligned himself with Connor, but quickly betrayed him. Diesel was ultimately a double-crosser and a bit of a coward that was corrupted with power.
• The Diesel Army (final appearance)
- Diesel's model was later turned into a scrap model. It was seen in The Road to Sodor.
- Diesel was the only known commander of the Diesel Army.