- "I believe in plain speaking. Speak up, the lot of you!"
- ―Duncan, Mavis and the Tornado
Duncan is a reckless yellow engine on the Skarloey Railway.
Duncan is famous for his "Rock 'n' Roll" swagger that he demonstrated when he first arrived to Sodor. He is a bit reckless and has been the cause of many accidents, especially at the Quarry. He has a history of being particularly rude to Rusty.
Duncan helped the Investigation Party uncover an abandoned gunpowder stockhold, complete with an impressive display of cannon weaponry. However, sparks from Duncan's funnel caused the floor to ignite and send an intense cannon bombardment through the walls and over to Ember Gorge, where Fearless Freddie and Sir Handel were situated. Since then, the Thin Controller has been very reluctant to trust Duncan with any sort of task.
Despite the Thin Controller's better judgment, Duncan was allowed to join the pursuit of Proteus to prove to Rusty he was not afraid of the spectre. Rheneas derailed during the pursuit, which meant the Investigation Party had to split up. Quickly moving into action, Skarloey went after the mystery spectre while a very spooked Duncan stayed back to help Rheneas home.
Duncan crashed into the Sodor Shakespeare Company's outdoor theatre, and the Skarloey Railway had to pay for the repairs. Duncan was chosen to play the role of Ophelia, a female character. This irritated him greatly, causing him to be quite short with all the other engine's joking during rehearsals. Duncan loses it when two audience members begin heckling him, which prompts him to storms off stage, claiming he has had it with this role.
Ducan is arrogant, rude, swanky, and a bit boastful, but he has shown that he can have a big heart. His attempts at impressing the others with his prowess usually backfire, and he has shown difficulty learning any sort of lesson from his mistakes. However, he is a tough little engine and really useful all the same.
- Season 0: Whistle Trouble
- Season 1: Lift Bridge (cameo), Dodge (remastered version cameo), The Old Warrior (cameo), Cannon Fodder, Greg and Lars (cameo), Conspiracy Theory (cameo)
- Season 2: Two Hearts Burn Together (cameo), Pigeon Hunting (does not speak), Scot-free (cameo), Mavis and the Tornado, Rosie (cameo), Just Another Wild Goose Chase, Rock-Star (cameo), Munitions (cameo), Blunderbuss, Swan Dive (cameo), Luke and Millie's Blue Mountain Adventure (mentioned), All the World's a Stage, Swashbuckler, Thomas and the Fortune Teller (cameo)
- Season 3: Samson, BoCo, and the Man in the Hills (cameo), Chivalry is Dead, Percy and Asbestos (cameo)