When it was aired here decades ago it was under a title that meant: "The locomotive Thomas and his friends".
The episodes aired now are titled "The Thomas train".
I always refer to the show as 'the locomotive Thomas', or just Thomas, same if I say something about Thomas himself. But when I talk about the locomotives in plural, then for some reason I tend to make the mistake of saying 'the trains'. I don't know why, since I know it's not correct use of the word.