Duncan is a Scottish narrow gauge engine. He initially arrived to the Skarloey Railway as a spare engine after Peter Sam had a nasty accident with some trucks that broke free from the incline. He was prone to boisterous bouncing about, which became known as "rock 'n' roll", whenever the mood took him, which once caused him to derail in the mouth of a tunnel. He speaks with a Scottish accent and is a rather tall engine. He often grumbles and complains, usually about not being polished or being over-worked, and can be rude, rough, and bad tempered. He claims to be a plain-speaking engine who believes in plain speaking, an approach which can cause him to make callous or belligerent remarks.
Duncan is based on the Talyllyn Railway's Douglas, an Andrew Barclay 0-4-0WT.