The SECR N class is a type of 2-6-0 ("mogul") steam locomotive designed in 1914 by Richard Maunsell for mixed-traffic duties on the South Eastern and Chatham Railway (SECR). Built between 1917 and 1934, it was the first non-Great Western Railway (GWR) type to use and improve upon the basic design principles established by GWR Chief Mechanical Engineer (CME) George Jackson Churchward. The N class was based on the GWR 4300 Class design, improved with Midland Railway concepts.
The N class was mechanically similar to the SECR K class 2-6-4 passenger tank engine, also by Maunsell. It influenced future 2-6-0 development in Britain and provided the basis for the 3 cylinder N1 class of 1922. Production was delayed by the outbreak of the First World War in 1914, and the first N class rolled out of Ashford Works in 1917, three years after design work was completed. The class replaced obsolete 0-6-0s as part of the SECR's fleet standardisation, as they used parts interchangeable with those of other classes.
Eighty N class locomotives were built in three batches between the First and Second World Wars. Fifty were assembled from kits of parts made at the Royal Arsenal, Woolwich, giving rise to the nickname of "Woolworths". They worked over most of the Southern Railway (SR) network, and were used by the Southern Region of British Railways (BR) until the last was withdrawn in 1966. One N class locomotive is preserved on the Swanage Railway in Dorset, undergoing overhaul.
Background Three factors dictated the type of locomotive that could run on the South Eastern and Chatham Railway (SECR): increased freight and passenger train loadings, poor track quality, and weak, lightly built bridges. An increasing number of passengers used the SECR to reach the cross-Channel ferries at Dover and Folkestone between 1910 and 1913, and heavy goods trains between Tonbridge and Hither Green marshalling yard stretched the capabilities of existing locomotives and infrastructure. On the lines of the former London, Chatham and Dover Railway (LCDR), flint beach pebbles on a bed of ash had been used for ballast. Conventional track ballast has irregular shapes that "lock" together to keep the track in place, whereas the smooth pebbles used by the LCDR failed to prevent track movement under strain. The economies in construction meant that only locomotives with low axle loadings could run safely on the track. These restrictions meant that the SECR was unable to follow a coherent locomotive strategy that reduced costs and increased serviceability. The railway's Operating Department had to use mismatched classes of underpowered and obsolete 4-4-0 and 0-6-0 locomotives because they could run within the restrictions imposed by the infrastructure. This meant frequent double-heading that increased operational costs.
Richard Maunsell was appointed CME of the SECR in 1913, following the retirement of Harry Wainwright due to ill health. Wainwright left a legacy of competent but unspectacular locomotives that struggled to cope with the increased train lengths and loadings. Maunsell took control of the short-term situation by improving existing designs, and he introduced new engines to progressively replace obsolete classes. New designs could also cut costs on the SECR, as one capable mixed-traffic locomotive could undertake the work of two separate passenger or freight types. The first new design was to become Maunsell's N class 2-6-0.
Design and construction
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 , 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.
SECR batch
Production of the first batch was delayed by the outbreak of the First World War. Assembly began towards the end of the war and the first locomotive, No. 810, emerged from Ashford Works for proving trials in July 1917, one month after the first K class tank, whose design was derived from the N class. Entering service in August 1917, No. 810 was trialled for three years before another 15 locomotives (Nos. 811–825) were ordered in 1919. These were built between 1920 and 1923; their construction delayed by a backlog of repairs caused by the war. The first left Ashford Works in June 1920, featuring a greater superheating surface area within the boiler as a result of operational experience with No. 810. All locomotives were equipped with tenders.
In 1922 modifications were made to No. 822 during assembly. This was because production delays at Ashford prevented the building of a proposed 3-cylinder design drawn-up in 1919. Maunsell and Holcroft revised No. 822's cylinder arrangement to accommodate a third inside cylinder fitted between the frames. The outside cylinders were also reduced to diameter to accommodate the inside cylinder and its associated valve linkages. The differences between No. 822 and the rest of the N class meant that this locomotive was re-designated as the 1919 proposal, becoming the prototype of the SR N1 class when completed in March 1923.
"Woolwich" batch The first batch of the N class proved successful in service, and few problems were encountered after settling-in. The Ministry of Supply drew up a contract for a second batch to the same specification – to be built at the Royal Arsenal, Woolwich. The government backing came as part of a proposal to nationalise the railways, which would require a standard fleet of locomotives to promote economies in production and maintenance. The nationalisation proposal was abandoned; instead, the government passed the Railways Act 1921, which grouped the railways into the "Big Four" in 1923. Building of the second batch went ahead to retain skilled labour at Woolwich, but the fabrication of 119 boilers for allocation to the kits of parts was contracted-out because of limited production capacity at Woolwich and Ashford; the North British Locomotive Company built 85, Robert Stephenson and Company 20, and Kitson & Co. 14. By 1924, the prefabricated components stored at Woolwich formed 100 complete N class kits for purchase from the government.
The newly created Southern Railway, which had absorbed the SECR in the 1923 "Grouping", undertook trials in early 1924 to compare the performance of its freight locomotives. Because the Ns were designed to haul both freight and passenger traffic, Maunsell, as the newly appointed Chief Mechanical Engineer of the Southern Railway, decided to compare the design with the N1, LSWR S15 and LB&SCR K classes in trials that involved hauling trains of 65 loaded wagons. Although the S15 was superior in freight haulage capacity and operational economy, the N class’ good all-round performance on passenger and freight meant that the type was adopted as the company's standard mixed-traffic design. The Southern Railway subsequently bought fifty "Woolwich" kits for assembly at Ashford between June 1924 and August 1925. These were identical to the SECR batch and were given numbers in the series A826–A875. The Midland Great Western Railway of Ireland bought 12 kits prior to its amalgamation into the Great Southern Railways, which bought an extra 15. The latter 15 locomotives were divided into eight GSR Class 372 with driving wheels and six GSR Class 393 with driving wheels: the final kit was kept for spares.
The Metropolitan Railway bought six kits for conversion to the Metropolitan Railway K Class 2-6-4T tank engines, which were similar in outline to the SECR K class. The remaining 17 complete kits at Woolwich were bought by the Southern Railway, and formed the basis of later locomotive classes such as the three-cylinder SR W class 2-6-4 tank locomotive. The prototype W class was produced in 1932 from N class parts with the addition of water tanks, a coal bunker, a rear bogie and a third cylinder between the frames. Woolwich also stocked a surplus of N class bogie components, and these were bought by the Southern for rebuilding the E1 0-6-0 tanks into the E1/R 0-6-2 tanks.
Southern Railway batch In 1932, the Southern Railway ordered a final batch of 15 locomotives (Nos. 1400–1414) to expand class availability on the Southern Railway's network. These were built at Ashford works and differed from the previous 65 in a number of ways. The cabs of the final eight locomotives (Nos. 1407–1414) were fitted for left-hand driving, which was adopted as standard by the Southern Railway. The original N class chimney was replaced with the lower-profile version used on the U1 class, which increased route availability by allowing the locomotives to pass under lower bridges and tunnels. Maunsell had begun to research smoke deflection techniques to improve driver visibility on the King Arthur class between 1926 and 1927, which resulted in the adoption of a standard smoke deflector design for the Southern Railway. A smaller version was fitted to Nos. 1400–1414 during building.