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.
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.