William is a steam-diesel hybrid engine who works on Sodor.
William was the brainchild of Rudolph Diesel, a compromise between the sensibilities of both himself and Sir Nigel Gresley. Upon learning he was cut from the project, Diesel discarded the plans and instead focused upon seeking vengeance for being left out.
Many years later, Phil discovers the plans amongst Sir Topham Hatt's things, at which point he summons Diesel back to Sodor to inquire about it. With some persuasion, Phil convinces Rudolph Diesel to help him construct the engine. They are close to completing the engine when Stephen frames Phil of building a weapon instead of an engine, which halts work on the engine. After Phil clears his name and sets things right, he and Diesel put the finishing touches on the engine. Upon its unveiling, Rudolph christens the engine "William." After being photographed with Phil, Diesel, and Robert, William gained a face. Three days after William's completion, Rudolph Diesel passes away peacefully in prison.
After having worked on the railway for some time, the other engines grew tired of William constantly talking about how fuel-efficient he was and how he never had accidents. He quickly made close friends with Henry, who attempted to help him fit in better, but to no avail. After narrowly avoiding an accident that S.C. Ruffey attempted to cause, William realized that the other engines were right about S.C. Ruffey being the main reason the trucks were so misbehaved. William then went to Phil and suggested getting rid of him. However, Phil could not do so because it would interfere with his ability to purchase ballast from Sam Ruffey. William eventually convinced Phil to allow him to hatch a plan to remove S.C. Ruffey from service. He buffered up to the truck from behind, under the charade that he was taking a recently-ordered and “long-overdue” special, then began pushing him at top speed. When he saw a set of points that had been set to go into a siding atop a cliff, William braked, causing S.C. Ruffey to shoot forward through the buffers and over the edge of the cliff. When Sam Ruffey came to inspect the damage, Phil told him that S.C. Ruffey was damaged beyond repair. Ruffey was still utterly depressed and told Phil his intentions to never do business with the railway again, a decision he would soon regret. Following the incident, the engines all praised William for getting rid of S.C. Ruffey.
More than a decade later, Ryan became depressed following the destruction of his coaches at the hands of Ruffey. He eventually decided to be painted blue so that he would fit in better, only to realize that Thomas had been the only blue engine on the railway for many years. William felt sorry for Ryan and, in an effort to cheer him up, decided to get himself painted blue as well, declaring that he and Ryan were bringing back the NWR’s blue livery. The two engines became close friends, to the pleasure of Phil. James, who was passing by, then outright admitted to William that he despised him and was glad he was no longer red.
- Season 3: Inheritance Part 4 (not named, does not speak), Inheritance Part 7 (does not speak)
- Season 4: Lineage (cameo), William, Chilling Point, Grand Unraveling, Insecurity
- William is based upon an experimental 2-6-2T locomotive built by Kitson-Still Company in 1934. The engine worked on trial for the LNER before being deemed ill fit for practical usage and was dissmantled.