A normal truck uses it's brakes constantly when going down hill, turning all that potential energy of the cargo into heat. This truck charges batteries to slow

A normal truck uses it's brakes constantly when going down hill, turning all that potential energy of the cargo into heat. This truck charges batteries to slow itself down via a generator. At the bottom, it weighs much less, thus even accounting for loses, there's enough energy to get the "lightweight" truck back to the top.