Dieselisation
Dieselisation took place largely because of the tremendous reduction in operating costs it allowed. Steam locomotives require large pools of labour to clean, load, maintain and run. They also require extensive service, coaling and watering facilities. This was their biggest inferior measure as compared to the diesel locomotive in the number of ton-miles or passenger traffic miles either could run. Diesels could and did have a significantly higher initial price per unit-horsepower delivered. However, their far greater range between fueling stops, the absence of water stops, and the much higher unit availability between inspection repair and maintenance stops, were far better than steam. Diesels simply required significantly less time and labour to operate and maintain. Diesel power was also more scalable to power requirements, owing to the control systems that allowed multiple units to be controlled by one operator. "Double header" steam power required a crew for each locomotive. Initially, diesel locomotives were less powerful than the typical steam locomotives. Between the late 1930s and the late 1950s the power available with diesel locomotive engines roughly doubled, although the most powerful steam locomotives ever built still exceeded the power of the most powerful diesel locomotives from the late Twentieth Century.
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.