This has been a fascinating thread. I served my time on B.R. as an engineering apprentice (mechanical), 1961-65, mostly in the shops at Crewe, and mostly on steam locomotives. I remember we did a light general repair on the last former LNWR freight 0-8-0, probably built 1920ish at the latest, in '61; a couple of years later I watched the frame of a 9F 2-10-0 that couldn't have been more than 7 or 8 years old cut into slabs with an oxy-acetylene torch. Some of the new diesel classes, notably the Metrovick Co-Bos, went for scrap before some of the steamers that they were supposed to replace . . . . I often wondered about the economics of such decisions . . . .
I also wondered about the relative efficiencies of steam versus diesel. A steam engine is self-starting; once you have pressure in the boiler, you open the regulator and she moves, but you need some kind of starting mechanism to start an i/c engine, either electric, compressed air or a hand crank (might be difficult with 2500hp!); also, the explosive combustion of fuel in an i/c cylinder produces far more energy than the engine can use, so anything up to 50% of the energy produced has to be dumped into a cooling system, and ultimately into the environment; a steam engine doesn't need a cooling system, so how come an i/c engine is supposed to be "more efficient"? David Wardale, in his book "The Red Devil and other tales from the age of steam", reports that his Cass 26 4-8-4, rebuilt from a SAR Class 25, had an overall thermal efficiency comparable to an equivalent diesel, and was actually cheaper to operate because of cheaper fuel costs per BTU, but the SAR had already decided that diesels were the future so they didn't want Wardale's engine. Politics, as elsewhere, overrode both engineering and economics.
Then there are turbines, of course. What would have happened, I wonder, if the LMS "Turbomotive" had been developed further? Steam turbines ran successfully in the U.S. and I think Germany, but I don't have any figures on those (Good Ol' Uncle Google probably knows . . .) Gas turbines are inherently inefficient since you use anything up to 75% of the turbine's output to drive the compressor. Some of the exhaust heat can be used to pre-heat the combustion air, but you're still left with a woefully inefficient machine; by all accounts, the gas turbine locos that ran on the BR Western Region burned something like three to four times as much fuel as the equivalent diesel.
So . . . . I wonder a bit, as usual.
And don't get me started of the line closures of the 1960s. I was there, so I know about some of the shenanigans that went on to make some services seem uneconomic, even when they really weren't . . . .