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.