In September 1924, Gresley approached Harold Yarrow (of Yarrow & Co.), and over the next three years they designed a new water-tube boiler for locomotive use. The resulting water-tube boiler consisted of a long steam drum, and four water drums. The water drums were connected to the steam drum with a series of tubes through which water circulated (see photograph):
Orders for the cylinders were placed in July 1928, along with the first parts of the Yarrow boiler. Although an official announcement of the "Hush-Hush" was still a month away, news of a mysterious new locomotive began to leak out of Doncaster Works. By February 1929, the boiler had been constructed and fitted to the smokebox. Simultaneously, Professor W.E.Dalby conducted a series of wind tunnel experiments which resulted in the unusual front end. This required some changes to the smokebox design. The boiler tests were completed by October 1929, and the partially-assembled locomotive was shipped to Darlington. To maintain secrecy, shipping over LMS lines required the locomotive to be sheeted up.
On 13th October 1936, the water-tube boiler W1 made its last journey: from Darlington Works to Doncaster Works to be rebuilt with a conventional fire-tube boiler. Out of the 1,888 days since it was built, No. 10000 spent 1,105 days in Darlington Works.
After a problematic career, the lone W1 was finally withdrawn on 1st June 1959.