Train Buses / Train Wheels / Road Wheels:
If the train wheels are between the rear road wheels, and the switch from one drive to the other is a clutch, or two, maybe even just one double sided clutch, on the drive shaft, or even on specially made road wheels the mechanism could be tripped driving on and off the tracks. Allowing the drive not in use to be in neutral gear is the objective. Specially designed wheels are hardly a hardship.
Include disc brakes. Automatic gear box obviously. High speed reverse on rails.
Just think of it. No more dragging luggage miles to and from a rail station. Just get on the bus at your door in Buckie or Banff, or anywhere on planet earth, pay your fare/s to the driver, and you’ll be in Edinburgh before you know it. Back even. Or off at the airport and fly to anywhere.
No more train stations. No more bus stations, those places we all hate. Just bus stops and timetables. What luxury. Drive on, drive off points here there and everywhere. There is no limit to the ease of travel it would provide.