Actually for a while there vehicles were designed to swap out the entire motive unit as well as the energy supply. You could even use different motive units for

Actually for a while there vehicles were designed to swap out the entire motive unit as well as the energy supply. You could even use different motive units for different purposes, such as long distance, larger loads, small journey etc. This technology was called the 'horse and carriage'. :-) But joking aside I do wonder whether exactly this would be practical with non-living motive units. With modern tech it wouldn't be hard to control a separate traction unit, with its own wheels, at the front of your vehicle, and you could swap an electric one for a fossil fuel (or rented larger battery one) for long journeys in exactly the same way. I guess it would just be hard to design in such a way that it fulfilled all the things we expect from a modern vehicle, such as high stability, low drag, maneuverability, etc.