It trails behind what car? I've only seen a few photographs of them and they are always by themselves. I would call them a "bot". Bus on track. Are we talking about the same thing?
Its a trailer because trailer cars/carriages often don't have motors or associated equipment and because they need motors in order to move, they can only trail the path of a motor vehicle. Some vehicles do have compressors, transformers or even pantographs, because there usually isn't space on the motor car to take them.
