[conservatism_IS_compassion]

Even if one were to produce a miraculous power source for a locomotive which could supply all the needed power and energy in a fifty pound package, one would still have to throw in many tons of balast to have a usable locomotive. By contrast, cars are supposed to be lightweight. If someone could design a car's power source to be 50lbs while still providing useful power and energy, that would be a major engineering coup.

All true. But at least so far as fuel economy is concerned, the energy recovery/storage of the hybrid is of course intended to ameliorate the energy loss otherwise associated with braking of a massive vehicle.

Diesel locomotives are all hybrids.

They are if in fact they include large batteries - a fact of which I have not heretofore been aware ( gogeo could be right, I suppose . . .).

[supercat]

They are if in fact they include large batteries - a fact of which I have not heretofore been aware

I don't think they have particularly large batteries--certainly nothing large enough to provide a useful amount of locomotive force, but they nonethtless all use the diesel engine to produce electricity, and then use that electricity to drive the wheels.