Duncan 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 can be a competitive engine too, particularly with James. He can be careless and make rash decisions which usually leads to his own downfall, but is not always quick to apologise or realise the error of his ways. However, upon Skarloey recalling Rheneas' gallant act to get his passengers home, Duncan admitted he had the wrong attitude and tried hard to be a more useful, respectable engine in future. In spite of his ways, Duncan can be a little misunderstood as he is focused, confident, resourceful, and has a big heart.