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.
East Coast Main Line
During the early 1960s, the line's steam locomotives were replaced by Diesel-electric counterparts, amongst them the purpose-built Deltic, a powerful high-speed locomotive developed and built by British manufacturing conglomerate English Electric. The prototype was found to have been successful in its trials, leading to a fleet of 22 locomotives being built and put into BR service to handle all the important express traffic. Designated as the , they were powered by a pair of Napier Deltic engines, which had been originally developed for fast torpedo boats; the unusual three crankshaft triangular configuration of the engines was the source of the locomotive's Deltic moniker. Their characteristic throaty exhaust roar and chubby body outline made them unmistakable and distinctive amongst their peers. The Class 55 was for a time the most powerful diesel locomotive in service in Britain, capable of providing up to .