Duncan was built by Andrew Barclay Sons & Co. at their Caledonia Works in Kilmarnock, Scotland to their simple and straight forward industrial design in 1928 and worked in a factory in Scotland, which Peter Sam claims is the source of his strong language and volatile temper. Duncan is powerful and can haul all but the heaviest trains; but his short wheelbase gives him a tendency to ride roughly at speed.
In 1958, Duncan was bought secondhand by Sir Handel Lloyd Brown as a spare engine, as the workload was too much for Sir Handel and Peter Sam to manage unassisted. He came soon after Peter Sam's accident with the slate trucks and was re-gauged to 2ft 3in. Duncan was rude and arrogant at first. However, following accidents involving the tunnel and a worn stretch of track south of Cros-ny-Cuirn, he began to see sense and made up his feud with Rusty, whom he had detested for being a diesel.
He was still selfish and prone to temper tantrums and actually stalled on Rheneas Viaduct on the claims that he was overworked and not polished enough. Skarloey, fed up with his careless attitude, finally taught him sense with the story of Rheneas' single handed running of the railway and how passengers are far more important than a polish.
When some boy scouts were camping and volunteering for the Skarloey Railway, they were going to help fix a part of the line that was wet and mushy when it rains. When the scouts became thirsty, the leader stopped Duncan to help them find something for them to drink in fear that they might be dehydrated. Unfortunately, the Refreshment Lady ran out of drinks since business was very busy for her, but Duncan suggested another tea shop near the station by the lake. They managed to pick up enough drinks, and the boys cheered Duncan for helping them. He even replied by saying "It's only fair we should help you too!"
Duncan has visited the Talyllyn Railway in Wales as a tourist attraction. He was there in 2005.
Duncan has also been very cocky, but this has often fired back at him.
Once, Rusty told Duncan a story about a a ghost engine who fell off the Old Iron Bridge one night. That next morning, Peter Sam's driver had an idea and tells Duncan's driver and fireman of a trick they can play on Duncan the following night for all his teasing, their chance soon comes, as Duncan was crossing the Old Iron Bridge the next night, he notices some fireflies forming the shape of an engine which spooks Duncan, then his driver dropped a rock into the ravine, which spooks Duncan even more and he goes to his shed scared. This put Duncan in his place, thinking he had just seen a ghost.
His ego once landed him in a swamp, after being careless working on the incline railway.
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.
In the Railway Series, Duncan is painted in the Skarloey Railway's red livery with blue-and-yellow lining. In the television series he is painted yellow with black and gold lining.
- Ryōichi Tanaka (Japan; fourth - seventh seasons)
- The Reverend W. Awdry's model of Duncan is currently on display at the Narrow Gauge Museum in Tywyn.
- Duncan's large scale model is currently on display at the Hara Model Railway Museum in Japan.
- Duncan's tendency to do "Rock-n-Roll" that resulted in incidents such as scraping tunnels with his cab and derailing were inspired from his real life basis Douglas, who was known for being a "Rough Rider" with its short wheelbase.
- Duncan is the only narrow gauge engine whose whistle sound has not changed since the fourth season.
- Duncan has had several modifications over the series. These include:
- Season 5 (large scale model only):
- The wrinkles on his forehead disappeared.
- His eyes and nose became slightly larger.
- His eyebrows became slightly thicker.
- His buffers became silver.
- Season 6:
- His silver buffers became black.
- He gained a visible whistle.
- Season 5 (large scale model only):
- "Who is Duncan?"
- "He came as a spare engine after my accident."
- "Is he useful?!"
- "He'll pull anything, and I'm sure he means well, but he's bouncy and rude. He used to work in a factory, and his language is often...strong."
- "I understand," said Skarloey gravely.
- Skarloey and Peter Sam talking about Duncan, "Home at Last", "The Little Old Engine".
- ERTL (normal and metallic; discontinued)
- Wooden Railway
- Bandai Tecs
- Brio (discontinued)
- Wind-up Trains