I wasn't going to get involved with this thread but here I am.... Proven boiler design gave an efficiency probably about as good as you could get so any improvements have got to be downstream of the regulator. For my two pennyworth I'd be looking at a shaft drive loco and getting rid of coupling rods altogether. Gearbox technology had improved significantly by the late 60's and I'm sure that our gearbox manufacturers could have produced a reliable and robust design suited to the requirements. Put the basic loco on bogies and add chevron suspension, too. The basic steam engine would be a multi-cylindered, short stroke affair on its own dedicated bed frame, probably fitted with roller bearings (but not necessarily). This would allow the bearing design to be dedicated to overcoming the thrust of the pistons. I always thought that early loco engineers had the right approach with this and it was Stephenson and his Northumbria loco design that condemned steam loco technology to having bearings that carried load and thrust for the next 150 years. It might even be a worthwhile thing going back to that early technology and to put the cylinders in the boiler so that they have a steam jacket although it would be a far more complex boiler design to produce. What else? As I've said, shaft drive would allow bogies and all the advantages they provide. Oil firing would allow operation from remote cabs. The technology already existed in the 60's so there would be no Leader type arrangement with a fireman divorced from a driver and in a remote hot house. Would all this be possible? I'm not sure!