Dieselisation
Dieselisation took place largely because of the tremendous reduction in operating costs it allowed. Steam locomotives require large pools of labour to clean, load, maintain and run. They also require extensive service, coaling and watering facilities. This was their biggest inferior measure as compared to the diesel locomotive in the number of ton-miles or passenger traffic miles either could run. Diesels could and did have a significantly higher initial price per unit-horsepower delivered. However, their far greater range between fueling stops, the absence of water stops, and the much higher unit availability between inspection repair and maintenance stops, were far better than steam. Diesels simply required significantly less time and labour to operate and maintain. Diesel power was also more scalable to power requirements, owing to the control systems that allowed multiple units to be controlled by one operator. "Double header" steam power required a crew for each locomotive. Initially, diesel locomotives were less powerful than the typical steam locomotives. Between the late 1930s and the late 1950s the power available with diesel locomotive engines roughly doubled, although the most powerful steam locomotives ever built still exceeded the power of the most powerful diesel locomotives from the late Twentieth Century.
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.