In 1929, Gresley attempted to build a high-pressure locomotive using the Yarrow boiler from shipbuilding. This boiler consisted of three barrels arranged in a delta shape and was not divided into a firebox and a boiler barrel. It was faired with a streamlined mantle and operated at 450 psi. The chassis came from an A1 Pacific and was expanded to include an additional, independently movable trailing axle. A four-cylinder compound engine was used, which had relatively small cylinders due to the high pressure. Since the boiler did not meet the expectations placed upon it, the locomotive was rebuilt with a conventional boiler in 1936 and was given three cylinders of the same size. 