 The size was constrained by the heavier axle-loading of Maunsell's proposed 2-6-4 tank locomotive variant of the N class, the K class, and was consequently smaller than was otherwise possible on the 2-6-0 chassis.The need to reduce overall weight also meant that the latter would feature lightly braced frames.Maunsell's Chief Locomotive Draughtsman, James Clayton, brought functional Midland Railway influences to the design, such as the shape of the cab and the drumhead-type smokebox, which sat on a saddle that was of wider diameter than the fully lagged and clad boiler.