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.
Electro-Motive Diesel
By the late 1940s the majority of American railroads had decided to transition from steam to Diesel power, known as Dieselization. The demand for Diesel-powered passenger service did not grow at the same rate as for freight service, as air and automobile travel were the burgeoning modes of passenger travel. Nevertheless, the lower profit margins of passenger service made the cost advantages of Diesel over steam all the more critical and steam power was increasingly seen as dirty, smelly, and dated among the travelling public. To meet post-war demands, EMD opened another locomotive production facility in 1948 at Cleveland, Ohio.