SECR K and SR K1 Classes - Design and Construction

Design and Construction

The 2-6-4 wheel arrangement was not in common use in Great Britain at this time, as many railway companies operated routes that required locomotives with greater fuel capacity, or short branch lines that necessitated smaller locomotives. The 2-6-4 tank engine design had only been used once before for standard gauge locomotives in Britain, on the Great Central Railway's 1B class freight locomotives of 1914. However, the configuration was ideal for the SECR, because of its shorter mainlines, and allowed for a long wheelbase with a leading axle to permit greater stability at speed on track curves. The tightness of the curves on the former LCDR mainlines had constrained the size of locomotives operating on the SECR, as they had been hastily erected during the nineteenth century to compete with those of the South Eastern Railway (SER). The longer locomotive could also accommodate a larger boiler than a 4-4-0, giving sufficient power to avoid double-heading of locomotives on heavier trains.

The K class design used a "Bissel bogie" leading axle and a plain trailing bogie. The trailing bogie permitted the use of a large coal bunker that was capable of sustaining the locomotive over the run between London Charing Cross and Dover Marine, and side water tanks of 2,000 imp gal (9,100 l; 2,400 US gal) capacity were used, negating the need for a tender. The coupled wheelbase between the rear and centre driving wheels was reduced from that used on the mechanically identical N class to 7 ft 9 in (2.362 m) to accommodate the bogie. The cab was fully enclosed, although the set of four small front spectacle plates (the windows on the front face of the cab for forward visibility) were the same as those used on the N class.

The K class was designed by Maunsell's team in 1914 as part of his proposed standardisation programme following the N class, but the designs were not shown to the railway directors until early 1915 to enable all six designs to be shown at once. The design incorporated the principles of power and reliability established by George Churchward, using a Belpaire firebox that sloped downwards towards the cab instead of a round-topped version, a regulator located in the smokebox, long-travel valves for free running at high speeds, a sharply tapered and domeless boiler, and a right-hand driving position. The inclusion of these features is attributed to Holcroft, Maunsell's personal assistant, who had worked on the GWR 4300 class and the N class. James Clayton, Maunsell's Chief Locomotive Draughtsman, brought simpler and more 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. The latter was fitted with Ross pop safety valves and pressed to 200 psi (1.38 MPa).

Other innovations by Maunsell's team included greater superheating surface area, locating the boiler water top feed inside a dome-like cover with external clackboxes and water feed pipes mounted on either side, outside Walschaerts valve gear, and parts that could be shared with similar locomotive classes to reduce maintenance costs. The firebox was narrower towards the rear and featured a continuously sloping grate, whilst the ashpan was fitted with front and rear damper doors, the latter adjusted to clear the rear driving axle. The lower part of the coal bunker incorporated a water tank of 760 imp gal (3,500 l; 910 US gal) capacity. This was connected to two 620 imp gal (2,800 l; 740 US gal) side tanks by two rectangular pipes on either side of the locomotive that also formed supports for the cab footplate.