So, it’s true that if trucks were fitted with massive electric motors (essentially making them hybrids), then the only part of the equation left is doing something with the generated electricity. Storing the electricity released by a laden truck going down a long hill would be impractical with modern batteries. Trains that get electricity from overhead power lines could put the juice back on the grid, for instance.

Other vehicles use a variety of systems to store some of the energy from braking; some use compressors to put air cylinders under high pressure, for instance. (Not to be confused with air brakes.) Those systems also couldn’t deal with the very large amount of energy involved here, though.

My goal was to build a system that did not require replacing the trucks themselves, but diesel-electric big rigs might start making sense in the future, which would certainly simplify things as you pointed out.

The engine(s) of the locomotive or truck doesn’t directly power the wheels. Rather, the engine creates electrictity that is either stored in batteries or used to power an electric motor at each of the drive wheels.

When it’s time to slow down, the circuit is reversed, so the motors turn into electric generators and slow the train or truck down, while charging the batteries.

Of course, this technology comes at a price, as people who bought Priuses three years ago are finding out when it comes time to replace the batteries. But for industrial applications, it’s quite efficient.

(Odd thought: I wonder what BNSF and Phelps Dodge do with all of their worn-out batteries? There must be somebody doing recycling on a colossal scale.)