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.
Dieselisation
Competition from diesel spurred a round of development in steam locomotive technology. High style, high speed "steamliners" produced during the second half of the 1930s became the speed kings of passenger service. Duplex and articulated steam locomotives built in the early 1940s exceeded the power of any diesel ever built, although their power was edged by gas turbine-electric locomotives during the 1950s. But the limits of steam technology were rapidly being reached. The new locomotives were mechanically complex and extremely specialized. Locomotive size became an issue, as steam engines became so big in the 1940s that the cylinder and boiler dimensions were pushing the limits that the loading gauge would allow. Fireboxes became so big that firing a steam locomotive became an extremely difficult job without the aid of mechanical stokers. Mechanical stokers for feeding coal to locomotives were in use in the 1920s. Steam turbine-electric locomotive power was developed in 1938 by General Electric. GE abandoned the project in 1943 after results were unsatisfactory during trials with three railroads, however, Baldwin Locomotive Works built three steam turbine-electric locomotives in 1948. They were expensive to operate and insufficiently reliable for their intended service, and were scrapped in 1950. Baldwin's steam turbine-electric program resulted in one more locomotive in 1954, which was also not successful in service.