At first glance, this looks like a dumb idea. It’s a diesel powered bus that can drive onto a railroad spur, lower flanged steel wheels, then ride the rails to a destination where it hits the streets again. When operating on the rails, power continues to be supplied by the diesel, with the rear tires riding on the rails for traction. The driver still has to accelerate and brake, but doesn’t have to steer while on the tracks. Because everything in Japan must be properly licensed, the operator must be certified both as a bus driver and railroad engineer.
At second glance, it looks like a dumb idea. The main reason for building this appears to be to give the existing railway track something to do after the train service was discontinued, attract that sliver of the tourist market eager to ride in a bus/train, and sell merchandise, including stuffed plush versions of the vehicle.
Yup! Vehicles similar to the Caltrain maintenance trucks are de rigeur for the vast lengths of freight rail lines in the Western US. Highly practical in areas with more roads than rail lines.
That does not necessarily mean the Japanese road/rail bus is a good use of the concept. In urban areas with overloaded roads and unused rail tunnels, it might provide faster rush hour transit than road-only buses. In other areas, maybe not so much.
A back of the envelope calculation looks like a potential 15-20% reduction in energy demand by operating on tracks vs roads.
A turning steel wheel in contact with a steel rail reduces by 85-99% the amount of rolling friction than a rolling rubber truck tire has in contact with an asphalt or concrete pavement. The train wheel’s reduction in friction over a car tire is even greater.
I’m not sure how much calculations based on hauling containers on trains vs. trucks are applicable to something like the Japanese bus/train. The motive power is still delivered by the rear rubber tires in contact with the rails, so while you get rid of the rolling friction of the front tires, you still have the friction of the powered rear tires. There is no reduction in frontal area and air resistance. The engine and power train are identical, so there’s no gain there. Braking is still entirely dissipative with no recovery by regenerative braking. In addition, there’s the added weight of the wheels and suspension for running on the rails and the mechanism to raise and lower them when changing modes.