I see my previous reply has been censored in a couple of places, it should say b then u then double g then ers on the end.
Probably the most efficient steam railway is that powered by electricity from a power stations via the national grid. A power station is most efficient at full capacity and will use the nearly same amount of fuel on full load or just waiting in standby to go on line. In this country we suffer from the problem that demand suddenly goes up through the roof when most of the country puts on the kettle for a cuppa when the ads begin when the favourite soap opera or show finishes on TV. Its probably better to improve the efficiency of one big unit than thousands of little ones, however there have been some funny experiments with steam loco's from using turbines instead of pistons, marine type water tube boilers, pulverized coal and oil burning, trouble is they never amounted to much and basically poorly converted conventional steam loco's with no real performance or operating benefits. The Americans only seemed to get it right with their Challenger and Big Boy locomotives, only one British steam loco broke the mould, O.V.S Bulleid's Leader design on the southern, it looked more like a diesel than a kettle, with fully welded boiler (a first), sleeve valves and chain drives and cabs on either end for the driver it looked promising especially if they could be made to work in multiple with one man at the controls. However a decision abandon oil firing and retain coal fuel meant the fireman had to work right up against the boiler backplate in a hot cramped isolated compartment, a passage between the cabs meant the boiler was off centre which lead to improper weight distribution and riding problems, the engine used sleeve valve technology as used in some aeroplane engines, however these needed superb lubrication with any deficiency leading to seizing and splitting of the setup, the fitters often left the inspection covers off and with inevitable entry of grit and dirt causing the same result. Only one was fully completed with the five others in various states of erection, with another one about to trials they were all quickly scrapped. Bullied had some very quirky ideas used on his other loco's namely the Pacific's, which might have been good in principle but failed in practice and all had to be rebuilt when BR took over, he went on to become chief engineer in Ireland and failed there too. The thing is steam loco's at the time were'nt very friendly to the crews that had to work them, unlike some of the German designs, there's a big difference between top express driver to night shift engine cleaner. Another thing was the culture of the industry was you started out as a cleaner and worked your way up and this also applied after the war when there was a labour shortage, so instead of training up younger men on the new traction a lot of the drivers put on the new type of loco's only had a few years left until they retired and were'nt about to change they're ways and with little or not training no wonder there was problems, it was not uncommon to see one driving a diesel with his head out the cab window all the way steam engine style, however they say that when the diesels came in there was as small sigh of nostalgia and a big sigh of relief.
Maybe a modern design of steam powered loco for off the wire work, would look more like a diesel or electric instead of the tradition steam loco but burn LPG, I know there was work done in Germany for a mountain railway, the steam loco for it size had more even torque over the rev range and was more efficient burning diesel fuel than a diesel engine. There were plans to build a modern steam loco (not the A1 peppercorn) but it never got of the drawing board.
By the way, Beeching was'nt the only one who had it in for the railways, Margret Thatcher had plans to get rid of it altogether, sell of the land to developers and there was even artist impressions of the new motorways going into London were once the tracks had been, B=$*h.