Duncan was built by Andrew Barclay at Kilmarnock, Scotland and worked in a factory in Scotland, which Peter Sam claims is the source of his strong language and volatile temper.
In 1958, Duncan was bought secondhand to Sir Handel Brown II 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 derailments in Rheneas Tunnel and on a 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!"
Rusty told Duncan a story about a ghost engine who fell off the Old Iron Bridge one night. His driver and fireman spooked him by making him think that there was a ghost engine trying to cross the Old Iron Bridge. He worked with Skarloey, Rheneas, and Rusty at the incline railway, but became very careless, resulting in him landing in a swamp. Duncan ignored Rusty's warnings about the old wooden bridge, and Rusty had to rescue him from falling into the river below. He ruined The Hot Air Balloon that Mr. Percival had hired to take his twins for a ride in. However, the balloon repair man managed to fix it by using their birthday flag.
Duncan is well known and often teased for his temper. He has met his match before, a passenger who grumbled more than him! He also showed a lack of Christmas spirit and often grumbled about the busy work he has to do during that time of year, even going as far to say "Bah Humbug!" Despite his rough exterior, he heroically helped Luke up a hill around Christmas time, showing his kinder side, and was rewarded with a new coat of paint.