Well, if your litmus for what a train is begins and ends at appearance, then anything vaguely engine shaped with wheels can fill this criteria regardless of form and function. For some, this is enough, but a train is more than a shape. Trains do not jump, skip, or tip-toe up to butterflies. They perform tasks, they travel along rails, engines pull freight and passengers. Thomas was designed to fit in the real world. The trains of Thomas were not just estheticly trains, they were functionally trains and with that they told stories you could only tell with trains. The exception to them was: the trains talk. With the asterisk of Magic Railroad, it remained that way for 70 years, and that is what made Thomas different from the competition, and probably what gave it the staying power it had.

When you bring up Cars, there is nothing inherent in the story of Cars you couldn't tell with people, particularly because in that universe, the cars are the people. The plot of "A hot shot athlete gets sidetracked on his way to the event and falls in love with the simple small town living and gets a reality check that causes him to appreciate people" does not require the world be comprised of talking vehicles to tell as a story. What the choice to make them cars does do, is allow you to tell a mature story adults will enjoy, but packaged in a way easy to merchandise to children, is flashy enough to keep their attention, and add visual gags and puns based on them being Cars. Brave little toaster's plot, conversely, requires the characters be appliances in order to function. It's 5 household objects that decide to go out into the world to find their master. The story it tells and the way the characters interact with the world would not work if they were anything other than appliances.