This high-pressure locomotive was notable because it was not constructed upon the Schmidt system, unlike most of its competitors. It was designed by the LNER’s Chief Mechanical Engineer, Sir Nigel Gresley, and its main aim was simply fuel economy.
It had a single steam circuit consisting of a Yarrow water-tube boiler working at 450 psi, based on naval boiler practice, with which Yarrow & Co were very familiar.
It was a compound design, with two inside HP cylinders and two outside LP cylinders, fed by Walschaerts valvegear.

No 10000 never received a name, but it was widely known as the "hush-hush" locomotive because it was built in some secrecy.

Some significant dates:

1924	Design work begins
July 1928	Cylinders ordered
Feb 1929	Boiler construction completed
Oct 1929	Boiler testing completed
Nov 1929	Locomotive completed
Dec 1929	First trial run
April 1930	Superheater area reduced
Oct 1936	Last journey to Doncaster for rebuild

Left: The 4-6-4 10000 The height of the HP steam drum meant that the chimney did not rise above the casing, and so the front was shaped to scoop air upwards and clear the smoke, following wind-tunnel tests conducted by Professor W E Dalby. The rectangular opening below the smokebox admitted combustion air which was, in theory at least, preheated by passing around the inner boiler casing.

Above: A contemporary drawing of the 10000, with naming of parts.

On its first run in January 1930 steaming was wholly unsatisfactory, the highest steam pressure recorded in the HP steam chest being 305 psi. An intermediate superheater (surely really a reheater) was added to raise the LP steam temperature by 100 degF. Other modifications such as a double blastpipe were tried, but boiler performance was apparently always poor, probably below 50%.

Left: The 10000 with attendants

In October 1936 the 10000 was rebuilt with a standard A4 Pacific boiler.

Left: The 10000 spent quite a lot of its time at exhibitions to drum up publicity.

Left: The 10000 emerging from Stoke tunnel and entering Ipswich station. Photograph probably taken in 1932. The 10000 was presumably arriving for the exhibition advertised above.

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