Diesel was the first diesel engine to arrive on the Island of Sodor. He is painted black and has six wheels. He first came to Sodor for a trial run, helping Duck. After an embarrassing mistake, Diesel spread mean-spirited rumours about Duck, but once The Fat Controller got to the bottom of the incident, he sent Diesel back. From time to time, Diesel was brought in for help when no spare engines were available. Diesel would usually antagonize the steam engines, and be sent away in disgrace for causing accidents.
Diesel eventually became a permanent resident of the railway, and continued to feud with the steam engines. However, his personality has begun to soften. He was silently thankful when Thomas replaced his water-contaminated oil, and agreed to help the steam engines to complete an important job for The Fat Controller. Diesel stole the special coaches commemorating Gordon's record express run, but later revealed it was only because his own shunting record had gone unnoticed. The Fat Controller and the engines felt sorry for him, and at Gordon's ceremony, they presented Diesel with a new motor to replace his faulty old one. Once, Diesel rolled in front of Thomas in a billboard photo by mistake. Thomas failed to tell Diesel about the second shoot, and Diesel did the other engines' work while they were gathered for the photograph, making him late for his own jobs. When Thomas realized what happened, he decided to make amends by finishing Diesel's incomplete jobs. Later, Diesel created trouble when he challenged Thomas to a race backwards around the quarry. This left a special unfinished, and Diesel willingly helped Thomas clear the mess and finish their job.
Diesel is based on a British Rail Class 08 shunter.