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.
The Railway Series
The most obvious theme relating to British Railways was the decline of steam locomotion and its replacement with diesels. The first real instance of this was in the book "Duck and the Diesel Engine" (1958) in which an unpleasant diesel shunter arrives, causes trouble and is sent away. This theme may have been visited again in "The Twin Engines" (1960), in which an engine is ordered from Scotland, and two arrive, implying the other went to Sodor with his brother to avoid being scrapped. The 1963 volume "Stepney the "Bluebell" Engine" explained that steam engines were actually being scrapped to make way for these diesels, and again featured a diesel getting his comeuppance. The book "Enterprising Engines" was published in 1968, the year when steam finally disappeared from British Railways, and was the most aggressive towards dieselisation and Dr Beeching's modernisation plan. It features yet another arrogant diesel who is sent away, an additional one who stays on the Island of Sodor, a visit by the real Flying Scotsman locomotive, a steam engine, Oliver, making a daring escape to Sodor, and Sir Topham Hatt making a declaration that the steam engines of his railway will still be in service.