^^^
OK gotcha.
I think an electrical engineer could probably explain this exactly but I think the laymen's understanding is that the electricity generated by a diesel generator as in modern diesel-electric type locomotives doesn't hold a candle to the instantaneous brake horsepower/torque/juice (or whatever the measure terminology is) of an electric alternator powered by direct overhead/3rd rail delivered current. It's apples and oranges. As for whether a diesel-electric locomotive has ever existed or will ever exist that distributes its' diesel derived generated electricity to individually powered coach axles as in an EMU type configuration - the answer to the best of my knowledge is no. I can't explain why technically, but I would imagine there would be no performance benefit to such a system. If the loco lacks the acceleration power for performance comparable to an electric multiple unit train, than distributing that inferior up-front power to traction motors throughout the train would have little to no advantageous effect. Again a physicist or electrical engineer could probably easily explain this better. To take your question further, you may be essentially describing a diesel multiple unit (DMU). DMU's in long consists have multiple points of diesel generation throughout the carriages and are distributed to multiple traction motors, but performance remains significantly poorer in regards to acceleration and top-speed as compared to electric multiple units for the aforementioned reasons.
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Alternately, where are we with battery-powered electric trains?
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Well I know Siemens and others have trains in their product line that claim some on-board battery off-wire range, but what it is and what applications it could apply to IDK. I suppose in theory a battery train could perform just like a battery bus, where a recharging juice station is placed at specific intervals/locales (or train stops) to re-energize the batteries. But for world systems that already have "hard-wired" electrification (OCS or ground 3rd rail) there is no long term advantage to ever switch considering the lack of dependability that it would come with, even considering the built in cost of maintenance for a fixed system. As for Metra, such a proposal in that direction would be met by me at least as a half-hearted endeavor where line electrification is what is actually needed, not just for performance and environmental benefits, but for bringing the system up to world-class modern standards.