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.
Steele v. Louisville &amp; Nashville Railway Co.
The diesel-electric locomotive assaulted the primacy of the coal/oil fired steam locomotive from every angle. First, it had a much higher duty cycle than any steam locomotive could match. Famously, the use of steam locomotives to attempt the achievement of the schedules for the crack passenger trains between Chicago and the West Coast, required no less than nine (9) complete changes of engine and tender between Chicago and Los Angeles/San Francisco. With no steam engine/tender set travelling more than 250 miles. Diesels would complete these long distance runs with the same motive power units. And the diesel units would only receive fuel and water at limited operating stops. So the number of in-cab crews necessary to operate any schedule would be dramatically reduced by the introduction of diesels