^^ Various manufacturers have been developing such a technology for trams/streetcars... I know DC was considering it because of a century-old ordinance banning overhead wires anywhere in the central city. They ended up passing a bill to allow exceptions on a case-by-case basis, but never on the Mall or Pennsylania Ave.
The net emissions benefits of electrification depends on the type of power generation used. ComEd uses 58% nuclear power, so we're good on that aspect. Exelon has lobbied vehemently for cap-and-trade, so they strongly see themselves on a track towards carbon-lite or carbon-neutral. Plus, since an electric train doesn't have to lug its fuel around, it's automatically more lightweight and therefore more energy-efficient than a diesel train, regardless of whether it's loco-hauled or multiple-unit.
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la forme d'une ville change plus vite, hélas! que le coeur d'un mortel...
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