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 regauged 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.
Duncan is currently visiting the Talyllyn Railway in Wales as a tourist attraction and is currently being overhauled. Sir Handel has been brought in to do Duncan's work until he is mended.