Douglas is the number 10 black tender engine. He is Donald's twin.
Donald and Douglas work together all the time, but they sometimes have a falling out. This was proven when the two crashed into Trevor's hay cart. Donald accused Douglas of pushing him into the cart while Douglas accused Donald of pulling him. The twins refused speak to each other. When Duck needed help in the shunting yards, Donald took the opportunity to get away from Douglas. Things started to go wrong, and eventually Donald wasn't looking where he was going and crash through some buffers into a ditch. Duck convinced Douglas to help his twin and the two made up.
Later, Castle Loch was to be reopened by Lord Callan, so Donald and Douglas took some equipment up to the castle. The two argued whether or not the monster inside the lake existed or not. Unfortunately for the twins, a landslide occurs and the two became stranded. The Fat Controller promised help, but soon night came. The twins saw a shape moving towards them on the causeway. Donald and Douglas were petrified until they realised it was only Harvey who had come to help. Soon, the twins got their equipment to the castle on time and Lord Callan called them Really Useful engines.
Once, some points were jammed and Gordon was sent to pick up the engineer to fix the points, but instead picked up the carpenter. Gordon reversed to Abbey hoping to collect the engineer, but Douglas was in the way. Gordon decided that Douglas should go to Wellsworth and Donald to go to Maron to collect the engineer, and Donald bring to engineer to Douglas, and Douglas bring him back to Gordon. The plan was carried out and the points were fixed; Gordon thanked the twins for helping him.
Afterwards, Harvey was sent to help the twins after a tree had been knocked down. Thomas, however, misunderstood the situation and accidentally sent Harvey the wrong way. Thomas was proud of himself for being so helpful until he saw that Donald and Douglas were stuck and needed Harvey's help. He found Harvey and gave him the correct directions needed to help the twins. Thanks to Thomas and Harvey, the line was cleared and the twins were on their way.
They helped with the construction of the Harwick Branch Line, delivering most of the heavy construction materials. When Douglas went over the unstable track bed that hides the tunnel into Calles Cavern it began to crack, and the workmen only just stopped Donald from going over it. The twins witnessed Rocky lifting The Pirate Ship out of the cavern, and helped deliver it to Arlesburgh Harbour. They later attended the branch line's opening ceremony.
Donald and Douglas are Scottish twin engines. Donald is numbered 9 and Douglas 10. They are practical and level-minded, but, despite their maturity, they enjoy playing practical jokes and have been known to impersonate one another in the past. When they arrived on Sodor, they caused the Fat Controller confusion, as only one engine had been expected. The twins' numbers allegedly slipped off on the way and neither could recall them; a ploy to prevent one of them losing their twin to scrap. Namely, it was Douglas who had not been expected. His salvation from the scrap-heap may have contributed towards his decision to later aid Oliver in his own daring escape from scrap.
Donald and Douglas are very able workers, strong performers, and handle any task well. They are fully capable of being firm with the other big engines, such as Gordon and Henry, and even the troublesome trucks know better than to try their tricks on them. They are both used to harsh winter conditions too. Coupled back to back, with a coach between their tenders, they are very capable of patrolling the lines, clearing them of the snow. The twins both pull their weight on the Main Line and both Duck and Edward's branch lines. They can enjoy a joke with any engine. They are happy to act as the brawn of the Fat Controller's steam-driven goods haulers. Between the two, Donald tends to be the charming, witty joker whereas Douglas is the smart, quick-thinker, calmer-natured one of the pair. Both display a keen sense of wit and the pair usually get to have the last laugh, more often taking the moral high ground. But that does not mean they will not be just a little bit smug about it when a deserving engine meets their comeuppance.
Donald and Douglas hate to be parted from each other, but, like most twins, they often have their arguments among themselves and can squabble with one another. They do, however, love playing practical jokes on the other engines, and they use to impersonate one another. They were fitted with nameplates to specifically end this element of their humour. Donald and Douglas used to also have a strong dislike of diesels. But over time their dislike for diesels has softened and they appeared to have grown accustomed to their diesel companions.
Douglas is based on a Caledonian Railway "McIntosh 812" 0-6-0.
Douglas is painted in the BR's black goods livery with red-and-white lining on his tender.