3-cylinder K1 class 2-6-4 number A890 River Frome for similar treatment. Maunsell enlisted the help of the former GWR engineer Harold Holcroft, who had been recruited by the SECR as one of Maunsell's assistants. At Holcroft's suggestion, the rebuilt locomotive was to feature a 2-6-0 wheel arrangement, enabling it to operate over the lightly laid track-work on the Eastern section of the Southern Railway, whilst allowing an increased size of boiler to those featured on the 0-6-0s. As the rebuild used similar parts to the N and K classes to aid mass production, the new locomotive also had many features previously found on the successful GWR 4300 class, following and improving upon Churchward's design principles for this locomotive type. The redesigned locomotive was re-designated U1, and retained Holcroft's variant of the Gresley conjugated valve gear used on the K1 2-6-4. To facilitate the rebuild, Maunsell used his past experience with the N1 class 3-cylinder locomotives as a template for the new design. The style of the new locomotive also demonstrated the Midland Railway influences of Maunsell's other assistant, the ex-Midland Railway engineer James Clayton. Construction details Holcroft valve gear The valve gear assembly originally designed by Holcroft for use on the N1 class regulated the timing of the valve events that admitted steam into the cylinder, increasing efficiency whilst reducing mechanical wear and the effects of hammerblow on the track. Holcroft incorporated a new method of driving the middle cylinder from the right-hand set of the two outside Walschaerts valve gear without the need to use a separate set of valve gear between the frames, saving overall weight. This was achieved by using the combination lever assembly that controlled the fore and aft movement of the valve spindle, which admitted "live" steam into the cylinder and ejected "spent" steam through the ports. Holcroft's process of driving the middle cylinder directly from the combination lever contrasted to the approach taken by Sir Nigel Gresley, who used an extension of the valve spindles on the outside valve gear to operate the middle cylinders of his locomotives. This variant therefore held an advantage over Gresley's design, as the Holcroft valve gear would be immune to variations in valve events brought about by heat expansion of the valve spindles and flexing of the conjugation levers when in heavy use. Locomotive A890 Work started on the conversion of the solitary 3-cylinder K1 class No. A890