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.
Diesel locomotives of Ireland
Although prototype diesel locomotives ran in Britain before World War II, the railways of both the Republic and Northern Ireland changed over much more rapidly from steam to diesel traction, in the 1950s than those in Britain, due to the island's limited coal reserves and (in the Republic) ageing steam locomotive fleet. The initial diesel locomotives for CIÉ were built and supplied by British railway locomotive builders (Birmingham RCW with Sulzer engines and AEI Metropolitan Vickers with Crossley engines), with notably poor results from the latter. From the early 1960s, locomotives with more reliable engines from General Motors Electro-Motive Division (now the independent company, EMD), of the USA, were adopted. In the late 1960s the Crossley engines were replaced by EMD 645 units in a major programme to re-engine the fleet. Since the early 1960s all new locomotives on the two Irish rail systems have been purchased from EMD, with the exception of three from Hunslet Engine Company of Leeds, England, for NIR in 1970.