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
In rail transport, dieselisation refers to the replacement of the steam locomotive or electric locomotive with the diesel-electric locomotive (often referred to as a "diesel locomotive"), a process which began in the 1930s and is now substantially complete around the world.
The replacement of either steam or diesel haulage with electric locomotives is known as electrification. Whereas the benefit of replacing steam traction is indisputable, there is some dispute as to whether it is best replaced by dieselisation or electrification. Electrification has a high initial capital cost but the operating costs are lower. The overall savings depend on the effect of the investment cost compared with the savings due to lower operational and maintenance costs and the influence of better acceleration and tractive effort on railroad throughput. These are obviously different for e.g. urban networks and very long-distance networks with low frequencies. However, many railway commentators are increasingly suggesting that the ability of railways to operate with electricity not produced from fossil fuels may offer a decisive advantage over diesel power.