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.
Streamliner
With power and reliability of new Diesel units improved with the 2000 hp GM 567 powered E3 model in 1938, the advantages of Diesel became compelling enough for a growing number of rail lines to select Diesel over steam for new passenger equipment. The power and top speed advantages of state-of-the-art steam locomotives were more than offset by Diesel's advantages in service flexibility, downtime, maintenance costs, and economic efficiency for most operators. The American Locomotive Company (ALCO), builder of the Hiawatha speedsters, saw Diesel as the future of passenger service and introduced streamlined locomotives influenced by the design of the E units in 1939. The replacement of steam with Diesel power was interrupted by the US entry into World War II, with a military premium on Diesel technology that stopped all production of Diesel locomotives for passenger service between September 1942 and January 1945.