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.
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Though it was apparent to knowledgeable people in the railroad industry that reciprocating piston driven steam locomotives might soon be replaced by diesel power, Lima and C&O set out to build the ultimate in high power steam locomotives and they succeeded. No diesel engine ever surpassed the output of these giants, which were among the heaviest steam locomotives ever constructed. The three-axle trailing truck supporting the firebox was unusual, carrying over 190,000 lbs, allowing the huge firebox needed for the high power. As it turned out, steam locomotives continued in service for almost another 20 years.