Diesel locomotive
Diesel locomotives offer significant operating advantages over steam locomotives. They can safely be operated by one person, making them ideal for switching/shunting duties in yards (although for safety reasons many main-line diesel locomotives continue to have 2-man crews: an engineer and a conductor/switchman) and the operating environment is much more attractive, being quieter, fully weatherproof and without the dirt and heat that is an inevitable part of operating a steam locomotive. Diesel locomotives can be worked in multiple with a single crew controlling multiple locomotives in a single train — something not practical with steam locomotives. This brought greater efficiencies to the operator, as individual locomotives could be relatively low-powered for use as a single unit on light duties but marshaled together to provide the power needed on a heavy train. With steam traction a single very powerful and expensive locomotive was required for the heaviest trains or the operator resorted to double heading with multiple locomotives and crews, a method which was also expensive and brought with it its own operating difficulties.
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.