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 Sodor 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 the 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.
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.