That's not what is happening here. It uses gravity to generate energy as the mass going downhill is significantly greater than the mass going back up. The mass

That's not what is happening here. It uses gravity to generate energy as the mass going downhill is significantly greater than the mass going back up. The mass of the load is used for "powering" the regenerative braking system (resistance is converted to electricity, stored in the batteries) when going downhill. The batteries are now charged when at the bottom. The heavy load is removed from the train. The generated power is used to take the train with significantly less mass back up to be refilled. There is no "perpetual" energy here. Basically the fuel is the load. It still takes energy to load and unload the payload. No magic here. It's just a function of how much mass can the train take downhill, how much regeneration occurs during that operation, and how much energy it takes to take the empty train back up to the top. Just as you can take a tiny regenerative toy car, give it a few little revving pushes on the floor (let's say 4 feet of regeneration), you can let it go and have it go 100 feet, as all that mass you used from your arm to charge the battery was transferred (obviously not 100%) to the battery. It's not perpetual motion. It's just converting mass into energy. Pure physics here. This is the same concept when using water, rocks, sand etc. and gravity for batteries. It's just a simple transfer of potential energy.