Duncan is a yellow narrow gauge tank engine. He is named after the Arlesdale Railway's controller, Mr. Fergus Duncan. He is good friends with Sir Handel, has a twin at the Talyllyn Railway named Douglas and is known to "rock 'n' roll".
Duncan is a yellow narrow gauge tank engine that works on the Skarloey Railway,he had many adventures like pushing Rickety up the Quarry Mine Tunnel,shunting trucks into a goods train and taking them up Gordon's hill. We first met him in Home at Last, which is not one of the Thomas The Tank Engine and Friends Wooden Railway Adventures, but we also met him in some wooden adventures.
- Season 1: The Trouble With Trucks (does not speak)
- Season 2: Skarloey & Rheneas Depart For Crewe
- Season 3: Rusty Rocks the Railway, Clumsy Culdee and Rickety's Revenge
- Season 4: Duncan and the Passengers
- Season 11: Around the Railyard in 64 Seconds, Duncan in the Dumps, Hank and the Hatt St Crossing (cameo) and BoCo and the Cocoa (cameo)
- Season 14: Millie's Castle Hassle and Freddie at the Ready
- Season 15: Philip Finds a Friend (cameo), The Maron Station Makeover (cameo),
- Since to Duncan's rock 'n' roll, he sometimes derails on bumpy lines.