Dieselisation
Weighing against the cost of, and inertia against, replacing the large investment that railroads had in existing steam power were the dramatic increases in flexibility and efficiency with diesel. The fastest and most powerful steam locomotives were faster and more powerful than diesels; however, their range of efficient operation was severely limited. Diesels pro-rate their fuel usage to the length of trains, which a steam engine cannot do. Multiple-unit diesel power is scalable to power requirements with one locomotive crew; steam power is not. A high speed Hudson steam locomotive is good for only one situation, high speeds on level grades. The diesel locomotive can be operated by a single person, with no need of a fireman to run the boiler (two person cab crews may be required for other reasons). Also, diesels use much less fuel and no manpower when idling, something locomotives often do. Diesels can be parked running for days unattended, whereas steam engines must be constantly tended to if not completely shut down. Bringing a steam engine boiler up to operating temperature is often regarded as both an art and science, requiring much training and experience. A diesel is much simpler to start and shut down.
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.