Not necessarily. Amtrak is currently investing in dual-mode locomotives: https://www.trains.com/trn/news-reviews/news-wire/amtrak-siemens-finalize-deal-for-new

Not necessarily. Amtrak is currently investing in dual-mode locomotives: https://www.trains.com/trn/news-reviews/news-wire/amtrak-siemens-finalize-deal-for-new-equipment-to-replace-amfleet-i-equipment/ https://www.trains.com/trn/news-reviews/news-wire/amtrak-sie... My understanding of how these work is that when not connected to wires directly, a diesel generator supplies electricity instead. So partial wiring is possible with these. Amtrak has some additional constraints (they can't run diesels under the Hudson River) but to some extent a lot of freight operators have similar restrictions but deal with them differently; due to ventilation concerns, most Western mountain tunnels only permit one train in them at any given time, and the crews have to carry respirators and oxygen in case the trains stall inside the tunnel. --- Britain also has trains that do this running with a program of incremental electrification: https://en.wikipedia.org/wiki/British_Rail_Class_802 https://en.wikipedia.org/wiki/British_Rail_Class_802 Once the electrification is complete, the equipment can be converted to full electric operation simply by unplugging the diesel generators.