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
The Great Western Railway introduced diesel railcars in the 1930s and the first British mainline diesel locomotive was built by the London, Midland and Scottish Railway in 1947 but, unlike elsewhere in the developed world, the transition away from steam was delayed during the early postwar years. The delay was driven by two economic considerations: the lower initial cost of steam locomotives for immediate replacement of the large number of locomotives worn out from wartime service, and a projected rise in the cost of petroleum relative to coal, a plentiful domestic resource. Nationalisation of the railways took place in 1948; diesel locomotives were first introduced on a wide scale following the Modernisation Plan of 1955. Poor reliability among the first diesel locomotives used in the Modernisation Plan caused it to be implemented at a slower pace while the problems with the locomotives were worked out during the second half of the 1950s.