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.
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 .