10000 (also known as the Hush-Hush due to its secrecy) was an experimental steam locomotive fitted with a high pressure water-tube boiler.

The rearward "firebox" area was wide and spanned the frames, placing the water drums at the limits of the loading gauge.

[1] The boiler was constructed and fitted to the frames by Yarrow in Glasgow, involving the rolling chassis being carried over the LMS, carefully sheeted over to avoid inspection by a rival railway company.

The first works photographs, with the boiler cladding in grey, were taken in Glasgow, with a wooden dummy centre driver and coupling rod added for the photo.

In UIC notation this wheel arrangement could be described as a 2′C1′1′ (or more fully, 2′C1′1′h4vS) as the two trailing axles were independent, rather than a four-wheeled bogie as for those leading.

Gresley incorporated an ingenious unique system for giving independent cutoff to the high-pressure cylinders using only two sets of Walschaerts valve gear derived from the outside cranks on the Von Borries principle[2] and using an inside half-length expansion link.

[4] The corridor tender was similar to the ten built in 1928 for those locomotives of classes A1 and A3 that were used on non-stop services such as the Flying Scotsman.

[5] When it was deemed that no further progress could be made, the locomotive was taken to Doncaster Works in 1936 and rebuilt with a conventional boiler and three simple expansion cylinders on the normal Gresley layout.

The tender was not rebuilt, but was modified slightly at the front so that the ends of the curved side sheets now finished 8 feet 2+3⁄8 inches (2.499 m) apart; it was also given streamlined plating at the top (which was removed again in January 1938) and a longer coal chute.

[6] After the rebuild, the water-tube boiler returned to Darlington for pressure experiments and space heating, before being broken up on 10 April 1965, six years after the rebuilt W1.

[10] During a works visit in May/June 1948, the corridor tender was exchanged for one of the non-corridor type, and it was given British Railways livery and renumbered 60700.

[7][5] In January 2020 Hornby Railways announced that it would be producing a model Hush Hush/W1 in both original and rebuilt forms in 00 gauge covering the locomotive's lifespan.