Thomas is not a train, he's a tank engine, which is a type of locomotive. There's a big difference. A "train", technically speaking, is what he pulls - coaches, freight trucks, etc. Thomas is not a train himself, but he can be a part of one. Taken as one entity, he is a tank engine, which carries its water in itself instead of in adding tender - which he also doesn't need to carry coal, because he's got a bunker to carry that too. So by calling him a "train", you're just shooting yourself in the foot.