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.
Dieselisation
Dieselization got a boost in the early 1930s from three factors: the development by General Motors and its Winton Engine Corporation subsidiary of diesel engines with vastly improved power-to-weight ratios and output flexibility; the desire of railways to find more cost-efficient locomotion for passenger service at the height of the Great Depression; and design innovations in rail equipment that reduced weight, making the contemporary diesel engines, which were low-powered by today's standards, viable for mainline passenger service. The mid-1930s saw the introduction of lightweight diesel-powered streamlined trainsets such as the Burlington Route's "Zephyrs" and Union Pacific's M-1000x "City" trains, which were diesel's first assault against the dominance of steam in mainline passenger service. During the second half of the decade, diesel locomotives with sufficient power for full-size passenger trains were developed and put into regular production. Improved GM diesel engines in 1938 increased power and reliability. Dieselization of passenger service gained momentum as the decade drew to a close and the first model of mainline diesel freight locomotive was on the market in 1940. Dieselization was especially attractive to western railroads, for whom the watering requirements of steam locomotives were a problem in vast stretches of the western interior. Coal-country railroads were generally reluctant to embrace diesel, a competitor to one of their main hauling markets, well into the 1940s.