Thinking the new build 47xx through: To me there is nothing like a locomotive working on the road and working fairly hard . Never mind the speed, preserved railways are simply not very long. Recent new builds have extended the envelope and can be large and as fast as possible. If you are setting about a new build given the insurance and safety standards without running on the mainline are such that it is not such a stretch to go the whole hog. To build anything insurance and safety requires an audit trail of the critical items*: - you might not need the design and calculations if it is demonstrably proven from experience "grandfather rights"- but those all those really important bits will all need some record to certifiy that the metallurgy, forming and assembly were right, the whole trail quite comparable to aviation standards. And to be able to state everything was done to main line standards is the best insurance you can get against the standards being tightened or simply someone quibbling. The 47xx would haul a good length of train which is critical to the economics of main line excursions. It would not be particularly fast but it would be good at keeping a round steady speed - very effective, as the French demonstrated, to keep a path. Having extra adhesion, better geared with 5ft 8in wheels and with it's proper boiler it would have a distinct edge - over 10% larger grate area compared with anything with a No 1 boiler - Hall, Grange , Saint, 28xx -and a indeed a bigger grate than a Castle's. It is known to been a thoroughly sound and successful design, quite capable of skating in from Reading at 80 mph. Both the designs to which it is related - the 43xx 2-6-0 and the King were thoroughly sucessful and well proven too. So, I think on the main line it would be as safe a bet as you could get to do fine - both commercially and practically. Thinking ahead the developing problems are likely to be are rising speeds to get a path on the main line and indeed clearances - the cylinders are GWR standard with a bore of 19 inches rather than 18 and not on a short wheelbase. Though with the side play on the rear axle not an exaggerated overthrow. However, the No 7 boiler for the 47xx is quite remarkably similar to the King's. This is simply an enlargement of the No.7 - longer grate, half a yard longer in the barrel and, critically, with 250 psi boiler pressure, 25psi more. It would be probably possible - and the insurers were prepared to cover an increase to 250 psi for"The Unkown Warrior" on a modern analysis of the original Patriot boiler design for 200 psi - to increase the boiler pressure and then fit smaller cylinders to gain route extra clearance. This is just what being done with the Gresley P2 2-8-2 which is building. With attention to the balancing and the sucessful testing of Tornado for a higher speed it ought to be possible to get authorisation for say 65 mph, hopefully 70. A better cab and a screw reverse might be desirable, a screw reverser - rather than the marvellous long reversing lever - was designed for the proposed second batch in the 1930s. And I assume this batch would have had a side window cab like the 38xx version of the standard GW 2-8-0. Two exchangeable cabs and a power assisted lever reverse should cover differences of opinion. Obviously more work - time and expense - but it would make sure of something particularly good to get the routes and the extra performance on an effectively narrowing loading gauge. A shortage of reliable and powerful locomotives is recurrently remarked on. Think of it as a Castle not as fleet but simpler and with more grunt. Notes. The 47xx with the No 7 boiler was Churchward's final design. Some have said the best ever GWR loco. On the prototype and under him the No.1 boiler was tried and found wanting. The Kings must in time need major boiler work. To build a No 7 boiler now would be a valuable rehearsal for at least firebox work later. The critical pressings - the throat plate and the black plate either end of the firebox - are practically identical. Standard but not fullwidth cylinders would decidedly help to get all the GWR two cylinder locos that might run mainline inside the platform edges. *Critical parts. The boiler, the brakes and everything associated with them. The drawbars and the wheel centres and everything attached to them. Some etc. eg the leading truck and the rods, crossheads.