Duncan is a Scottish narrow gauge engine with rough manners and a strong temper. He was prone to boisterous bouncing about, which became known as "rock 'n' roll", whenever the mood took him, which once led to wedging in the north mouth of Rheneas Tunnel (probably caused from keystones knocked down by his funnel) and derailing on a stretch of line near Cros-ny-Cuirn. While Duncan means well, 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, (such as getting in arguments or short lived feuds with Rusty) 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. He is always willing to lend a helping hand whenever he can.
Duncan is based on the Talyllyn Railway's Douglas, an Andrew Barclay 0-4-0WT. This engine is currently selected to be dressed up as Duncan on special events associated with the "Thomas the Tank Engine" franchise.
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 golden yellow with black lining and gold boiler bands. He has brass, originally black, name and numberplates on the sides of his boiler and cab respectively.
- Season 100 - Samson's Waste Dump (cameo), Coach Brakes, Jealous James, Scottish Engines, Rheneas' Risky Ride (cameo), Duncan's Dilemma, and Rusty Rescue
- Tom Stourton (UK/US; eighteenth season onwards)
- Ryōichi Tanaka (Japan; fourth - seventh seasons)
- Chikara Osaka (Japan; ninth season onwards)
- Krzysztof Szczerbiński (Poland; eighteenth season onwards)
- Grzegorz Kwiecień (Poland; Emily Saves the World only)
- Stig Henrik Hoff (Norway; eighteenth season onwards)
- Paul Disbergen (The Netherlands)
- Fabrice Trojani (France and French speaking Canada)
- Marcos Patiño (Latin America; eighteenth season onwards)
- Cosmin Petruț (Romania)
- 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.
- When the Skarloey Railway returned in the sixteenth season, Nitrogen Studios went to the Talyllyn Railway to take measurements of the locomotives to make CGI models of the Skarloey engines. Unfortunately Duncan's basis; Douglas was having an overhaul and could not be measured, hence why Duncan did not return in Season 16. When Duncan actually did return in Season 18, he is clearly based on his television model not his basis like the other engines. He also does not have a removable Talyllyn-styled lamp like engines 1-4. Interestingly he got a brakepipe when none of the other narrow gauge engines have them.
- 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 9:
- Tail lamp.
- Season 18:
- Smaller in size than both his original model and his basis.
- Red running board.
- Lamp, lamp irons, brakepipes and outlets.
- Slightly thinner funnel.
- Slightly smaller dome and windows.
- Smaller boiler and coal bunkers.
- Brass nameplates.
- Shorter and wider sand dome.
- Sanding gear under the cab.
- Riveting under his coal bunker.
- The lining on his cab and bunkers became outdented and unpainted
- His eyes, nose and eyebrows returned to how they were in the fourth season.
- His cutout windows gained frames and glass.
- Season 100:
- Silver coupling hook and lamp iron.
- Grey brake pipe.
- His side-rods become silver.
- His rivets changed from black to grey.
- Season 5 (large scale model only):