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Regarding diesels vs. steam. There's the statement that a diesel electric can use its full power at low speeds. If this were true, road slugs would not be a thing. For those that don't know, a road slug is a typically a an otherwise road worthy locomotive whose prime mover has become worn out and has been removed. Rather than scrap them or part them out, some railroads, like Norfolk Southern, will full the empty space with ballast, usually concrete. In some cases the cab and full hood height will be retained. In others, like pic related, will have their hoods cut down for visibility. Cab equipped slugs are quite popular with crews since they lack the vibration of a complete locomotive with a prime mover on board Excess power is fed from the complete locomotive to the road slug(s) at low speeds for additional traction. This is because if full power were fed to the traction motors at low speed it would quickly overheat and fry them. If road slugs are used in a mainline consist once the train is moving at higher speeds power to the road slug will be cut as the locomotive's traction motors can make full use of the power its prime mover can generate.
