Purists will call it semi-streamline, but in 1929 its curved shrouding seemed revolutionary. It was built three years before the Flying Hamburger, five years before the Commodore Vanderbilt and has a distinction of being the direct predecessor of LNER A4 Class - the fastest steam locomotives on the face of the Earth!
The experimental Gresley W1 No. 10000 "Hush-Hush" was the only 4-6-4 tender locomotive to run in Britain*. It included a number of experimental features including a marine type water-tube boiler working at 450psi. Although this original design is usually dismissed as a failure due to the revolutionary water-tube boiler, No. 10000 did successfully work a number of high profile trains. It never carried a name, although it was often referred to as the "Hush-Hush" due to the initial secrecy of the project.
At the end of 1924, the American Locomotive Co. (Alco) Works at Schenectady produced the world's first main line locomotive with a high-pressure water-tube boiler. This was a 2-8-0 goods locomotive called No. 1400 Horatio Allen, which was built for the Delaware & Hudson. This used two-cylinder compound expansion, and had a 350psi boiler. Due to the similarity with marine practice, the Clyde shipbuilders of Yarrow & Co. were consultants to Alco on this project. Development of the W1 also started in 1924, when Nigel Gresley was considering ways of efficiently generating steam which used less coal than the (then) heavy coal consumption of the A1 Pacifics.
Gresley's earliest design using a water-tube boiler was from April 1926, and was for a 4-6-2 express passenger locomotive. This was intended to be comparable to the A1 Pacifics, and would have had three-cylinder compound working. This design would have had no space on the footplate for the crew. It was quickly modified by shortening the front of the boiler, and moving the furnace forward. Cylinder sizes were changed, and a fourth cylinder was added. Darlington recommended replacing the trailing wheels with a two-axle bogie, but this was not immediately acted upon. By 1927, the boiler pressure had been increased from 350psi to 450psi, and the requirement for the low-pressure cylinders was reduced to 200psi. On seeing an outline drawing in June 1927 which showed excessive rear over-hang and heavy loading on the trailing axle, Gresley instructed Darlington to change the wheel arrangement to 4-6-4. Technically, the rear bogie was partially articulated, with the first axle in an A1-style trailing axle but with restricted movement, and the rear axle was a true pony truck. Hence, many claim the W1 was technically a 4-6-2-2, rather than a Hudson (Baltic) 4-6-4.
With the addition of the extra axle, Yarrow recommended that the firegrate was extended. Initially Gresley added 7.5in, but this was removed due to concerns about footplate size. Fitting a water-tube boiler into the restrictive British loading gauge was proving a problem.
The locomotive was completed in November 1929, and first ran on 12th December. It then experienced six months of trials (including modifications), before entering traffic.
After further tests, No. 10000 "Hush-Hush" entered service on 20th June 1930. It reached the 70,000 mile mark in May 1933, and it entered Darlington Works for its first general repair. The opportunity was used to apply further modifications. The blastpipe orifice was reduced from 4.75in to 4.25in, and its height lowered by 2.5in. Condensation in the low-pressure cylinders was a problem whilst the engine was standing. Chapelon recommended that the exhaust from the high-pressure cylinders should be re-superheated before being reused in the low-pressure cylinders. Gresley accepted this recommendation, and fitted an intermediate superheater of 153sq.ft. This added about 100F to the temperature of the low-pressure steam.
Other modifications made at this time included the addition of aluminium foil insulation between the boiler and the casing; and the addition of circulation tubes connecting the front and rear water drums of the boiler. The locomotive returned to service in June 1934.
W1 No. 10000 ('Hush-Hush') entered Darlington Works again on 21st August 1935 for further repairs, having run about 90,000 miles since new. A number of further modifications were being considered, when Gresley ordered all further work to stop whilst he considered a scheme to rebuild it with a conventional fire-tube boiler.
No. 10000 was moved to Doncaster for rebuilding on 13th October 1936. An outline design based on an A1 pattern boiler stretched to proportions similar to P2/2 Wolf of Badenoch was produced by 24th November 1936. This design had three 20in cylinders, a boiler pressure of 250psi, and A4-style streamlining. When No. 10000 finally emerged from Doncaster in November 1937, it closely resembled this outline plan.
As much as possible of the original locomotive was kept, including the frames which had to be shortened by 18in. The trailing wheel arrangement was left unchanged.
No. 10000 retained its number under the Thompson 1943 renumbering scheme, but was renumbered to No. 60700 under the British Railways renumbering scheme of 1948.
* Text: The London & North Eastern Railway (LNER) Encyclopedia