- "Alright, number 10, you sulking iron wretch..."
- ―Carlo Debris to Diesel 10, Rock Star
Diesel 10 is a sinister, evil and peculiar diesel under the jurisdiction of the Sodor Logging Company.
A long time ago, Diesel 10 pursued Lady, an engine privately owned by Burnett Stone, and caused an accident. Years later, Diesel 10 arrived on Sodor (along with his two evil minions Splatter and Dodge) during the same time as Lady when she came to visit, and Diesel 10 decided to take out some of his own personal anguish from his past by destroying Lady. After a great deal of trouble, including falling off of a broken viaduct, he was going to be sent away but was leased to Carlo Debris instead. Lady went home with Burnett, but still comes back from time to time. In his new adventures, Diesel 10 now works at The Lumberyard for Carlo Debris, much to both of their annoyances.
When Splatter and Dodge returned to Sodor on trial, they completely failed to accomplish their jobs and they too were exiled to the Lumberyard, where they were greeted by their former boss, who told them he missed them and was happy that they were finally reunited before laughing evilly.
Carlo recently sent him to help out with the rockslide that blocked The Flying Scotsman's special passenger excursion. The Flying Scotsman was very curious and amazed by his claw! Diesel 10 stated the claw was not used for clearing rock, and recommened they try out Thumper.
Diesel 10 overheard a troubled Derek questioning his beliefs, quite interested in this development of a possible ally, though he himself noted that he didn't catch Derek's full self-introspective.
Diesel 10 is an evil, conniving, bullying, sinister, devious, ruthless, and psychotic monster who hates all steam engines and would do anything to have them scrapped. However, he has a helping side shown when he requested that Thumper clear the rocks on the line in Rock-Star.
- Season 1: Lift Bridge (cameo), Dodge (remastered version only), Snow Blind (remastered version only), Shepherd's Pie (cameo), Wilbert the Lumberjack (cameo), Cannon Fodder (cameo)
- Season 2: Grim Messengers of Doom (cameo), Mavis and the Tornado (cameo), Rosie (cameo), Rock-Star, Pummeling Percy (cameo), Swan Dive (cameo), Henry and Kurt (cameo), Swashbuckler (cameo)
- Season 3: Culdee Fell (cameo), Derek and the Two Faced Engines
- Web Clips: Mike and Frank Banter: SLOTLT, ScarletFire, and EE93's Return (cameo), Thomas the Tank Engine 70th Anniversary Celebration! (cameo), Walter Sliggs - Meet the Characters! (does not speak)
He will receive his own episode in Season 3.