Donald is a Scottish twin mixed-traffic engines, He has a twin named Douglas.
Donald and his brother Douglas are twins. Donald hates to be parted from his brother, and loves to play practical jokes on the other engines. Donald and Douglas love to play practical jokes, especially impersonating one another. They were fitted with nameplates to specifically end this element of their humor. 
Donald and Douglas were built by John F. McIntosh at the St. Rollox Railway Works in Springburn, Glasgow, Scotland and worked on the Caledonian Railway in Scotland, a large network that extended along the country's east coast as well as the central and south-western regions. Despite being limited to Caledonian metals, they appeared to have a familiarity with other railways, as they were familiar with the locomotives and livery of the Highland Railway.
Around the dusk of World War I, the Caledonian Railway needed two locomotives to assist the goods trains coming from the harbour near the Firth of Forth to Edinburgh. The Dubs & Co. were approached to build 2 extra 812 Class locomotives. They worked well until the CR was nationalized into the LMS. Their work was taken over by Fowler Class 4F locomotives and later in the Great Depression, Stanier Class 5MT and 8F locomotives. They were relocated to Glasglow where they handled stopping passenger/goods trains until 1958 when diesels came and took their work. They were threatened with scrap, and their only hope was to flee and emmigrate to the North-West of England....
In 1959 Sir Topham Hatt needed a goods engine and by his request, BR transferred 57646 (Donald) to Sodor.
To Sir Tophan Hatt's surprise though, two engines arrived. Donald's twin Douglas (57647) could not stand to be separated from his brother, and was smart enough to realise he would eventually be scrapped if he stayed in Scotland. Neither engine could be told apart: both claimed to have forgotten their numbers, and indeed their numberplates had "slyly slipped off" during the journey from Scotland, forcing Hatt to keep both until he could determine which engine was freeloading on his hospitality. It was later revealed that this piece of skulduggery had been planned not only by the two engines, but also by their drivers and firemen, who were all related.
The Fat Controller was unaware of Douglas' motive and threatened the two that whichever engine he found out to be the truant would be sent home. Both engines resolved to give outstanding service so that Hatt would want to keep both of them. This plan quickly went to pieces, however, after Douglas accidentally shunted Thomas' special coach, full of passengers, into a siding at Tidmouth, causing a great deal of fury for the passengers. Things were more complicated when Donald crashed into a signalbox at Tidmouth and Douglas destroyed the Spiteful Brakevan.
Despite all this grief, the twin's professionalism, competency, strong work ethic, and personable natures had begun to win them support, particularly when they demonstrated an aptitude for snowplough work that they had learned in Scotland. After rescuing Henry from just such a snowy ordeal, the other engines came over to Donald and Douglas' side which ultimately led to the engines of Sodor taking industrial action. After a suggestion by Edward, and being pushed into it by Gordon, Percy was presented to the Fat Controller as a deputation, arguing the twins' case on their behalf and pleading that, if sent away, they would be scrapped. Hatt's reaction to this was mixed - though he understood the engines' feelings he did not approve or appreciate them trying to interfere with his decision-making. However, Percy's statement and the twins' excellent performance was enough for Hatt to take pity on the twins and keep both. He demonstrated this intent with an offer of a fresh coat of paint for both engines, and by their request they were painted NWR blue.
Donald and Douglas are practical and level-minded characters who nevertheless enjoy a joke, especially impersonating one another - the fitting of the twins with nameplates was done to specifically end this element of their humour. Their tendency for jokes, however, has continued and both display a keen sense of wit and, where necessary, an acid tongue. In their work, however, their performance is exemplary and they can be described as some of the most well-regarded engines in service on the railway, comparable with Duck and Edward in their reliability.
Donald and Douglas understandably held a grudge against Diesels for a time - after all, the oil-guzzling engines were taking over their work - but after BoCo helped them in Edward's absence, they have agreed that there is nothing wrong with Diesels on the whole.
Basis EditDonald is based on Caledonian Railway "McIntosh 812" 0-6-0s so is Douglas.
Donald and Douglas were painted black from construction and arrived on Sodor in the BR's black goods livery with red-and-white lining on their tenders. They remained as such (with the addition of numbers on their tenders) until it was decided that both engines could stay, at which point they requested to be repainted in the NWR's blue livery with red-and-yellow lining. They apparently like the colour, as it reminds them of the Caledonian Railway, where many of their shedmates carried a similar shade of blue.
They carry two builder's plates on the sides of their cabs.
- The Reverend W. Awdry only made a model of Donald for his model railway; there never was (or is) a Douglas model. The model is currently on display at the Narrow Gauge Museum in Tywyn.
- According to the magazines, Donald and Douglas' drivers are related.
- In the third season, Donald and Douglas' nameplates were moved from their smokeboxes to the middle of their boilers.
- In one illustration of Branch Line Engines, Donald had a nameplate above his face.
- The twins' original numbers are an in-joke. The 812 Class numbers only went up to 57645.