Donald and Douglas are 2 fictional steam locomotives from The Railway Series books by the Rev. W. Awdry. They also appear in Thomas & Friends, the television series based on the books. They are twin Scottish engines.
The Fat Controller originally only ordered one engine from Scotland, but was surprised when two engines arrived, neither with numbers or any distinguishing features – both engines were painted completely black. He could not tell which engine was which, and so decided to keep whichever one was the more useful. Unfortunately, Douglas accidentally shunted Thomas' Special Coach, full of Passengers, into a siding at Tidmouth, thus infuriating the Passengers. However, the twins proved their worth to the railway in winter, when their experience and expertise with snow made them excellent snowplough engines. The other engines appealed to the Fat Controller in "The Deputation" and he decided to keep both of them. In the Railway Series, they were painted blue shortly afterwards and given nameplates.
The twins are almost identical (except for their names and numbers), and some episodes of the television series have depicted them as being inseparable (notably the episode "Twin Trouble"). The reason for their both arriving on Sodor was that Donald was bought and saved from scrap, but Douglas was not and was due to be cut up (this fact is revealed in the story "The Missing Coach"). They could not bear the thought of one being saved and the other not so they decided to go to Sodor together and hope for the best. As their crews were also related, it was not difficult for them both to stay on the island. To allow the engines to be told apart, the Fat Controller ordered that numbers be painted on their tenders. Following an incident in which the engines swapped tenders, he decided that it would also be wise to give them nameplates on their boilers as well.
Donald and Douglas are popular among the other engines, not only for their versatility but also their practical and friendly demeanour. They enjoy a good joke, as exemplified when Duck's driver slipped a duck into Donald's tank, resulting in Donald gaining a new friend in Dilly the duck. They put their all into any job, no matter how difficult. And, because of their own brush with scrapping, they have the utmost sympathy for engines escaping the scrapyard. It was this sympathy that made Douglas a hero, when he rescued Oliver and Toad (and an autocoach named Isabel in the books) from scrap and brought them to Sodor. In the Railway Series, Toad became his brake van following this incident.
The Twins are mainly used for goods work, but can be used for all sorts of jobs all over the railway. They are very much the "odd job engines" of the line. They carry out track repair, collect engines from the Works, rescue engines that have broken down or crashed and, of course, clear snow in the winter. The railway would not be the same without them. They are, in the Fat Controller's words, "Really Useful Engines".