Douglas is a 0-4-0WT built in 1918 by Andrew Barclay and Co. Ltd. for the Airservice Construction Corps.
From 1921 until 1945 he worked at the RAF railway at Calshot Spit, Southampton. After a period in store at Calshot he was bought in 1949 by Abelson and Co. Ltd. who presented him to the Talyllyn in 1953. After overhaul and alteration from 2ft to 2ft 3in gauge, he entered service in 1954 and was named "Douglas" at the donor's request. Although smaller than the other locos he has performed well and was returned to service in 1995, having been fitted with a new boiler, turned out in his old Air Ministry Works and Buildings livery.
Douglas spent many years painted red and blue and running under the guise of Duncan.
In 2013 he returned to service after an extensive overhaul. While he was out of service Sir Haydn took on duties for the children's day by running under the guise of Sir Handel.
Douglas is currently painted red with black lining. In his cameo in the Railway Series, he wears the same green livery as Talyllyn and Dolgoch.
- Douglas' cab being so tall that it could scrape the tunnels he ran under could be the inspiration for the story of Home at Last. Additionally, Douglas was known for being a rough rider because of his short wheelbase. This could have been what inspired Duncan's "rock 'n' roll" tendencies.
- Very Old Engines (cameo)