LNER W1 Hudson 10000

Almost all 4-6-4 steam locomotives used in Britain have been Baltic tank engines with one notable exception: experimental London and North Eastern Railway (LNER) W1 Hudson tender engine No. 10000.  It was designed by LNER chief mechanical engineer Sir Nigel Gresley in 1926 to test the Yarrow water-tube boiler, which promised better fuel economy than the fire-tube boilers used on the vast majority of steam locomotives.  The central steam drum was connected to four water drums mounted below, which altogether was almost too wide for the British loading gauge.  This boiler was built by the John Brown shipyard in Sheffield and completed at Yarrow in Glasgow, which required it to be shipped in great secrecy over the rival LMS Railway.

No. 10000 was completed at Darlington Works in 1929 after much modification, and came out of the factory as a streamlined three-cylinder compound, with two central high-pressure cylinders feeling two external low-pressure cylinders.  Never officially named, No. 10000 acquired the nickname Hush-Hush for the secrecy surrounding its development, and Galloping Sausage for its shape, though there were plans to name it either British Enterprise or Pegasus.  This nameless locomotive was painted in a ghostly "works grey" livery rather than the green livery used for LNER passenger engines.

Testing began in the fall of 1929 and continued into the spring of 1930, during which No. 10000 was found to produce a tractive effort of 32,000 pounds, which was about the same as an A1/A3 Pacific (4-6-2) such as No. 4472 Flying Scotsman.  Testing also found No. 10000 to be a poor steamer, as the water-tube boiler could not raise as much steam as the fire-tube boiler.  The locomotive was often in the workshop for repairs and modifications, typically at the rate of twice or thrice a year, and Gresley consulted French locomotive designer Andre Chapelon for help in 1933. 

Among Chapelon's recommendations were re-superheating the steam from the high-pressure cylinders, which addressed the problem of condensation in the low-pressure cylinders while the locomotive was standing still, and replacing the funnel with a Kylchap exhaust system developed by Chapelon and Finnish engineer Kyosti Kylala.  This exhaust system had recently been installed on Gresley P2 Mikado (2-8-2) No. 2001 Cock o' the North, and was also installed on No. 10000 in 1935.  In 1936, Gresley ordered No. 10000 back into Darlington Works yet again, this time to rebuild it with a fire-tube boiler, a three-cylinder simple-expansion system, and an exterior "Bugatti" casing to match the new streamlined A4 Pacifics such as No. 4468 Mallard.

Back when it had been the water-tube boiler testbed, No. 10000 had been weak and unreliable, spending 1105 of its 1888 days of existence in Darlington Works.  After being rebuilt with a fire-tube boiler, the locomotive found its performance and reliability greatly improved, with a tractive effort of 41,437 pounds, which was possible thanks to the large boiler.  Unlike other Gresley engines, No. 10000 was not renumbered by Edward Thompson in 1943, though it was renumbered to 60700 by British Railways after the 1948 nationalization.  This unique locomotive was retired in 1959 and scrapped, though its corridor tender was transferred to A4 Pacific Union of South Africa, which is still operational today.  This model of No. 10000 in its original configuration is on display at Darlington North Road Station's Head of Steam railway museum.