There have been various devices of this type proposed over the years and they are nonsensical or have been found not to work.
The following are examples:
- A Japanese rail-bus that was announced with great fanfare a few years ago and then sank into virtual oblivion.
- The Road-Railer system for towing road trailers in a train. Virtually abandoned now as the world's last operator, Triple Crown Services of Indiana, US, have just downgraded to a single service on a single route.
The reason these technologies did not take off and the reason this system will be a failure is due to the fundamental differences between road and rail. Road vehicles are not designed to tow a large weight. The heaviest road vehicles in NZ have a payload of a mere 62 tonnes (HPMV). That is a huge load for a truck but a tiny load for a train.
To make up a train load of vehicles then they have to be made very strong and therefore heavy in order to withstand the mechanical forces and stresses of a whole lot of these vehicles being coupled together and hauled along the tracks at 100 km/h. When the train speeds up or slows down, or goes up or down a hill, the stresses from the weight of all the vehicles coupled together is huge. The result is a rail wagon has to be very heavy, and in order to carry a decent payload on a rail wagon, the resulting axle loading is much higher than on a road vehicle> This is why rail tracks and bridges are built so strongly. For example a train can weigh from 500 to 2000 tonnes in New Zealand.
If you put a road trailer together that can be towed in a train of say 20 or 50 other vehicles of the same type then it will be so heavy that it will be seriously limited in the amount of payload it can carry in order to be able to run on average roads in New Zealand. This system says nothing about whether the wheels are carried all of the time for rail. Hi rail vehicles, of course, serve dual duty with their wheels being the same for driving. The second issue for a train coupling is that the vehicle will also have to carry the airbrake equipment needed for the train's continuous braking system. It will also have to carry the train buffers which will stick out of both ends of the vehicle.
The rail-bus system that was being investigated in Japan failed for a similar reason. These were not going to be operated as trains, they were going to be single vehicles. The problem is the extra weight involved in carrying the extra set of wheels for the rail track. So that idea has largely fallen out of favour now.
The document above references a Fraunhofer Auto Tram. That is something entirely different and due to the failure of the Japanese proposal it remains to be seen if it will really take off. But in practice they appear only to be designed to couple in a small number of units at a time which means it is not really a train.
The only way you can make a system like what is proposed work, for freight, is with a small number of units. The problem is, with a small number of units, you are not going to get any significant efficiency gain over roads - or any gain at all. Rail is designed for, and works best with, heavy loads. A small load is pretty inefficient. There wouldn't be any real advantage.