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.
To this would also be added that there was simply no in-cab motive power operational role for the "Fireman" of a diesel locomotive. The firemen had no engine-operation functions in the cab of a diesel locomotive. He had no functional responsibility to ignite, raise or lower, the power output of the diesel prime mover. This was fully (electro-mechanically) automated, under the sole control of the locomotive Engineer, from the introduction of diesel prime movers in locomotive service. It made all Fireman positions redundant when diesel motive power was planned. This fact gave railroad management; and their labor union's leadership pause; in devising a strategy for the implementation of technical innovation known as diesel locomotives.