Duncan is a Scottish narrow gauge tank engine, who came to the Skarloey Railway as a spare engine after Peter Sam's accident with the trucks. He is a grumpy and short-tempered engine who enjoys complaining about everything.
Duncan was built by Andrew Barclay Sons & Co. Ltd. in 1928, and worked at a factory in Scotland, where he picked up his volatile temper and coarse language. In 1958, Duncan was purchased by the Skarloey Railway secondhand, as the line needed another engine to help Sir Handel and Peter Sam with the increasing workload.
Upon his arrival, Duncan was bouncy and rude, and had a tendency to "rock 'n' roll" along the line due to his short wheelbase. This led to him derailing and becoming wedged in the tunnel at Rheneas when his tall cab scraped against the tunnel mouth, and he had to be rescued by Skarloey.
After being rescued from the tunnel, Duncan began to detest Rusty and became determined to send him packing. He ignored his warning about a bad bit of track near Cros-ny-Curin and began "rock 'n' rolling" along the line again, but when he reached the bad bit of track, Duncan derailed and came off the rails, and had to be rescued by his passengers and Rusty. Duncan learned sense, and when he returned to the sheds that night, he apologized to Rusty and befriended him. (RWS; The Little Old Engine)
However, Duncan was still prone to throwing temper tantrums when things did not go his way, and was cross when Nancy did not have the time to polish him after polishing Skarloey. Duncan, believing that the passengers and the other engines were getting their own special treatment in favor of him, became jealous, and deliberately came to a halt on the viaduct with a full passenger train, refusing to move. In the end, Skarloey had to come and take him and his train back home.