Donald and Douglas (originally known by their old numbers, 57646 and 57647, respectively) are a pair of Caledonian Railway mixed-traffic tender engines from Scotland. Douglas tagged along with Donald to Sodor in order to escape from scrap, despite Donald being the only engine expected. After proving themselves to be really useful, they were both allowed to stay. They now work as utility engines on the North Western Railway; working wherever they are needed. They work primarily on the Little Western, but sometimes on Edward's Branch Line and the Main Line. 

Donald and Douglas are Scottish twin engines. They are practical, peppery, proud and level-minded, but, despite their maturity, they enjoy playing practical jokes and have been known to impersonate one another in the past. They were fitted with nameplates to specifically end this element of their humour.

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 Sir Topham Hatt's steam-driven goods haulers. Both are smart. Between the two, Donald tends to be the charming, witty joker whereas Douglas is the quick-thinking, 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 occasionally have their arguments among themselves and can squabble with one another. Also, Donald and Douglas used to 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.