I think it originally came from the fact the book series dates to the 1940s. The earliest ones actually predate the 1946 nationalization of British railways.

I think it originally came from the fact the book series dates to the 1940s. The earliest ones actually predate the 1946 nationalization of British railways. Diesel locomotives were strange and disruptive to the steam-based world that was the baseline. What gets me is that a tiny island with only trivial industry has acquired an incredibly random fleet of locomotives. In practice, you'd expect to see either a small number of engines built as a fleet, or still relatively similar engines built over time with incremental enhancements. I could sort of rationalize it if it were being ran as a "hobby business"-- not necessarily based on economic needs, but because the operators are just pathological locomotive collectors, perhaps collecting what British Rail shed at scrap prices as it dieselized.