Dieselisation
Weighing against the cost of, and inertia against, replacing the large investment that railroads had in existing steam power were the dramatic increases in flexibility and efficiency with diesel. The fastest and most powerful steam locomotives were faster and more powerful than diesels; however, their range of efficient operation was severely limited. Diesels pro-rate their fuel usage to the length of trains, which a steam engine cannot do. Multiple-unit diesel power is scalable to power requirements with one locomotive crew; steam power is not. A high speed Hudson steam locomotive is good for only one situation, high speeds on level grades. The diesel locomotive can be operated by a single person, with no need of a fireman to run the boiler (two person cab crews may be required for other reasons). Also, diesels use much less fuel and no manpower when idling, something locomotives often do. Diesels can be parked running for days unattended, whereas steam engines must be constantly tended to if not completely shut down. Bringing a steam engine boiler up to operating temperature is often regarded as both an art and science, requiring much training and experience. A diesel is much simpler to start and shut down.
Steele v. Louisville &amp; Nashville Railway Co.
The diesel-electric locomotive assaulted the primacy of the coal/oil fired steam locomotive from every angle. First, it had a much higher duty cycle than any steam locomotive could match. Famously, the use of steam locomotives to attempt the achievement of the schedules for the crack passenger trains between Chicago and the West Coast, required no less than nine (9) complete changes of engine and tender between Chicago and Los Angeles/San Francisco. With no steam engine/tender set travelling more than 250 miles. Diesels would complete these long distance runs with the same motive power units. And the diesel units would only receive fuel and water at limited operating stops. So the number of in-cab crews necessary to operate any schedule would be dramatically reduced by the introduction of diesels