SECR N Class - Design and Construction

Design and Construction

For detailed information on numbering variations, see: Livery and numbering

The N class was designed by Maunsell in 1914 to provide a sturdy mixed-traffic locomotive with high route availability. Intended to replace several obsolete 0-6-0 types, the N class was the first step in the SECR's fleet standardisation programme, which also included the K class 2-6-4T passenger tank locomotive. Maunsell enlisted the help of former GWR engineer Harold Holcroft, who suggested that a 2-6-0 wheel arrangement would allow the class to run on the poor-quality track in north Kent. This arrangement allowed for a longer wheelbase with leading axle to permit greater stability at speed on tight track curves, which had constrained the size of locomotives operating on the SECR. A longer locomotive could also accommodate a larger boiler than an 0-6-0, giving the N class sufficient power to avoid double-heading of locomotives on heavier trains.

The N class 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 up to 70 mph (110 km/h), a sharply tapered and domeless boiler, and a right-hand driving position. These features are attributed to Holcroft, who worked on the GWR 4300 class before joining the SECR. The boiler was intended to become a standard component for use on future SECR locomotive designs, thereby reducing building times and improving organisation at the works. 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. Clayton was also responsible for the tender and chimney designs. Snifting valves were provided to prevent vacuum formation in the cylinders when the locomotive was stationary, and the outside Walschaerts valve gear incorporated single slide-bars and piston tail rods. Innovations added by Maunsell's team included steam-powered locomotive brakes, locating the boiler water top feed inside a dome-like cover with external clackboxes and water feed pipes mounted on either side, and a new type of superheater that segregated saturated and superheated steam. Maunsell also incorporated a screw reverser to control valve events, which was easier to maintain than the complex steam reverser configuration of previous SECR designs. All components were standardised for interchange with similar locomotive classes to ease maintenance and reduce production costs.