The whole fuel-electric idea, initiated by GM to get around the cost of building mechanical gear trains for the 2000-10,000 HP level, gets its efficiency from the idea that you use only as many engines hooked to generators as needed to provide the power needed for load and grade by the traction motors. Thus you might fire up five diesels to accelerate, three for a grade, one or two for cruising on the flat, and let just one idle on a downgrade while you use the traction motors for braking, dumping their energy into ballast resistors to be dissipated as heat.
Hybrid concept does it differently, engine large enough for cruise power, batteries to provide extra power for acceleration or short grades, and maybe some energy recovery during retro braking if the operator can appropriately adjust driving style.
Tom Test
Itasca Spirit 29B