Duncan is a yellow narrow gauge engine.
- Season 2 - Sir Handel Comes Home (cameo) and Peter Sam's Prickly Problem,
- Season 3 - Pop Special and Little Old Twins
- Season 4 - A Blast from the Past and Passengers and Polish
- Season 5 - Gallant Old Engine and Steamroller (cameo)
- Season 6 - Home at Last
- Season 7 - Mountain Engine
- Season 10 - Duncan Gets Spooked, Special Funnel,