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.
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 .