Ground-based vehicles can definitely be used as offensive weapons. Plenty of examples of people not in vehicles being injured. The truck used in the attack in Nice, France is a very clear example. Granted, it wasn't a transit vehicle, but there's no reason why one couldn't be.
Your claim was that taxis, buses, and trains couldn't be weaponized, not that there was a psychological difference between them. A bus or taxi can be run into a building as well. (I'll grant a train is different.)
