Duncan is a narrow guage engine.
Season 1 - Luke and the Special Party, Rusty and Victor (cameo), and Sleepy Merrick
Season 2 - Yellow Rheneas, King of the Mountains, Duncan and Smudger, and Luke and the Cuban Repairs
Season 3 - Duncan's Donuts and Duke Returns (cameo)
Season 4 - Peter Sam's Delivery (cameo), Special Twins
Season 5 - Skarloey, Rusty and the Snow, Which Way?, Duncan and the Ghost Engine, Toby and Rheneas (cameo), Duke rescue to Sir Handel (cameo) and Hiding in the Hills.