Duncan is a narrow gauge tank engine who came to the Skarloey Railway after Peter Sam's accident with the slate trucks. He is the main tritagonist of the narrow gauge engines (after Skarloey and Rheneas) Allies: Thomas, Edward, Henry, Gordon, James, Percy, Toby, Duck, Donald, Douglas, Oliver, Emily, Stepney, Terence, Skarloey, Rheneas, Sir Handel, Peter Sam, Rusty, Duke, Freddie, Mighty Mac, The Thin Controller (boss), Elizabeth, Madge, Colin, Proteus, Luke, Owen, Merrick, Derek, BoCo, Spencer, Mike, Rex, Bert