Duncan is a grumpy Scottish narrow gauge engine who came to the Skarloey Railway as a spare engine after Peter Sam's accident with some slate cars.

Duncan is a Scottish narrow gauge engine with rough manners and a strong temper, making him the least social of the Skarloey Railway fleet. He was prone to boisterously bouncing about, which became known as "rock 'n' roll", whenever the mood took him, which once led to wedging in the north mouth of Rheneas Tunnel (probably caused from keystones knocked down by his funnel) and derailing on a stretch of line near Cros-ny-Cuirn.

While Duncan means well, he often grumbles and complains, usually about not being polished or being over-worked and can be rude, rough, stubborn, and bad-tempered. This can easily be seen from his dislike for Christmas for some reason. He claims to be a plain engine who believes in plainly speaking, an approach that can cause him to make callous or belligerent remarks. Duncan can be a competitive engine too, particularly with James. He can be impulsive, even more so than Sir Handel, and make rash decisions which usually lead to his own downfall, (such as getting in arguments or short-lived feuds with Rusty). He 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 readily admitted he had the wrong attitude and tried hard to be a more useful, more social, respectable engine in the future. When Rusty helped him after his derailment near Cros-ny-Cuirn, he apologised to him and extended a wheel of friendship, proving he has gratitude.

In spite of his rough ways, Duncan can be a little misunderstood as he is focused, confident, resourceful, and has a big heart. He is always willing to lend a helping hand whenever he can and wants to be really useful.