The SECR N1 class was a type of 3-cylinder 2-6-0 ('mogul') steam locomotive designed by Richard Maunsell for mixed traffic duties, initially on the South Eastern and Chatham Railway (SECR), and later operated for the Southern Railway (SR). The N1 was a development of the basic principles established by the Great Western Railway's (GWR) Chief Mechanical Engineer (CME) George Jackson Churchward and by Maunsell's previous N class design.Scott-Morgan (2002), p. 18
The N1 prototype was the result of modifications made to N class No. 822 during construction in 1922. The locomotive became operational in 1923 and used parts interchangeable with other Maunsell locomotive classes. The prototype N1 was the only member of the class constructed before the SECR became part of the Southern Railway at the Grouping in 1923, and featured Holcroft conjugated valve gear, Holcroft's experience in this field later aided in the development of the Gresley conjugated valve gear which differed only in deriving the drive for gear from the valve spindle instead of the combination lever. The class set the precedent for the Southern Railway's subsequent 3-cylinder designs.
The N1s compared favourably with the N class, although the type showed little improvement in performance. More of the class were built when it became clear that the type's smaller cylinders provided greater route availability. A total of six engines were built. The N1 class was primarily used on the Eastern section of the Southern Railway network, and used by the Southern Region of British Railways (BR) from 1948. The N1s gave valuable service until they were withdrawn in 1962. None were preserved.
Background Richard Maunsell's N class mixed-traffic 2-6-0 prototype for the South Eastern and Chatham Railway (SECR) was completed in 1917. It showed a marked improvement in performance over his predecessor Harry Wainwright's 0-6-0 and 4-4-0 designs when tested on freight and local passenger trains. The success of the prototype encouraged the SECR management to order a batch of 15 in 1919 after government restrictions regulating locomotive production during the First World War were relaxed. Ashford locomotive works was already committed to undertaking deferred repair work however, which temporarily slowed construction of new locomotives. This resulted in the gradual completion of the 1919 N class order as works capacity permitted between 1920 and 1923.
Operational experience with the N class prototype demonstrated that it was capable of coping with peacetime passenger and freight traffic on the mainline between London and Ramsgate, but Maunsell anticipated an increase in traffic that would require a more powerful locomotive capable of hauling longer trains. The use of larger engines was prevented by low permitted axle loadings on parts of the SECR network, caused by economies in track construction by using flint beach pebbles as track ballast; this material failed to hold the track in place when under strain and could not support heavy locomotives.Bradley (1961), p. 47 In the meantime Maunsell settled upon producing a 3-cylinder version of the N class 2-6-0, which provided the opportunity to test a variant of the Gresley conjugated valve gear developed by his assistant, Harold Holcroft.
Design and construction
Maunsell and Holcroft completed plans for a new 3-cylinder locomotive in 1919.Whitehouse & Thomas (1992), p. 41 Designated N1, the new design was an attempt to increase the operational flexibility and power of the SECR 2-6-0 without substantially increasing axle loading.Casserley (1966), p. 436 Holcroft's previous employment as an engineer involved with the GWR's 4300 class meant that the N1 used the same Churchward features as the N class to aid standardisation and the manufacture of parts. These included the use of long-travel valves for free running at speed, right-hand drive in the cab and driving wheels. The retention of the 2-6-0 wheel arrangement meant ample accommodation for the N class' tapered boiler with Belpaire firebox.Clarke (2008), p. 40 As with the N class, detail design on the N1 was left to another of Maunsell's assistants, James Clayton, who brought functional Midland Railway influences to the design: the loading gauge-friendly shape of the cab, the tender and the large-diameter smokebox.Bradley (1961), p. 48 The smokebox housed a large superheater, regulator valve and snifting (anti-vacuum) valves.Casserley (1966), p. 35 The N1 also retained Walschaerts valve gear on both outside cylinders.
The main design differences with the N class included the cab front, exhaust arrangements, and the addition of an extra (inside) cylinder between the frames to drive the centre driving axle. Clayton had revised the layout of the cab front spectacle plates (small windows on the front of the cab) to improve forward visibility. This incorporated two large single panes of glass either side of the boiler instead of the four smaller windows used on the N class.Haresnape (1983), p. 39 The inside cylinder was to be actuated by Holcroft's design of conjugated valve gear, which consisted of mechanical links fitted to both sets of outside Walschaerts gear, eliminating the need for a separate set of valve gear between the frames to operate the inside cylinder.Railway Engineer (1923), p. 140 The result would be a reduction in weight and the amount of mechanical equipment in this inaccessible part of the locomotive.
Holcroft's valve gear design was also an attempt to address the problems associated with Gresley's conjugated valve gear, which was prone to variations in valve events caused by heat expansion of the valve spindles within the pistons.Holcroft, (1946), pp. 145–147 The design utilised the motion of the outside valve rods (the rods transmitting the motion of the driving axles to the valves, such as the combination lever) instead, although the restricted space between the back of the outside cylinders and the front driving wheels made it impossible to locate the rocking arms controlling the conjugating mechanism in the vicinity.Rowledge (1976), p. 11 Holcroft's solution was to move the mechanism to a position above the pony truck and ahead of the cylinders and link it to the outside valve rods with extension levers, which had to be accommodated within the strict SECR loading gauge tolerances. This necessitated a new design of cylinder block whereby the outside steam chests were set inwards and the size of the cylinders reduced to stroke and diameter. The sides of the outside cylinders consequently sloped inwards towards the top in a configuration similar to GWR 2-cylinder locomotives, thus providing ample space in which to accommodate the extension levers.Holcroft (1965), p. 95 The inside cylinder and conjugation assembly were set at an incline of 1 in 8 to clear the front pony truck, leaving space for a third set of independent Walschaerts valve gear between the frames should Holcroft's conjugation mechanism prove unreliable in service.Holcroft (1965), p. 96
Another issue concerned the strength of the locomotive frames around the cylinders.Holcroft (1965), p. 94 Taking inspiration from the 3-cylinder locomotives designed by Vincent Raven for the North Eastern Railway, Holcroft suggested that all three cylinders and associated steam passages should be cast in a single block, with rectangular holes cut into the frames to enable the outside cylinders to project through them. However, the SECR's established practice of using outside steam chests on locomotives with outside cylinders prevented this, and the design team resorted to cutting a gap in the frames to accommodate the outside cylinders, which were then secured in place with a bolted splice plate. Production limitations at the Ashford works foundry also prevented the manufacture of a single casting that incorporated all three cylinders.Rowledge (1976), p. 10 This meant that the cylinder castings were split into two sections, with the left and inside cylinders forming a single unit that would be bolted onto a separate right-hand cylinder casting. Holcroft's new cylinder block design also incorporated a separate saddle casting, a configuration that allowed the N1 cylinder patterns to be used with any diameter of smokebox, creating a standard component with potential applications on future locomotive designs.
Prototype The construction of the prototype at Ashford railway works was approved by the SECR management to commence in 1919.Middlemass, (1990), pp. 148–154 However, Ashford works was heavily engaged in tackling a backlog of repair and maintenance work caused by government restrictions during the First World War with little spare capacity for new construction projects, particularly when approval for the construction of the N1 prototype coincided with a priority order for 15 2-cylinder N class locomotives. In consequence, the works backlog meant that only 12 N class locomotives were completed between June 1920 and October 1922.
Seeing little sign of an increase in production capacity at Ashford, Maunsell decided to construct the prototype N1 locomotive from parts intended for use on the next N class locomotive in the batch, No. 822. The outside cylinders were exchanged for the N1 type, which had been machined at the GWR's Swindon railway works following casting at Ashford to quicken construction once the N class frames were modified to accept the new design of cylinder block.Railway Engineer (1923), p. 141 This produced a robust chassis capable of supporting both sets of Walschaerts and the Holcroft conjugated valve gear.Railway Engineer (1923), pp. 140–143 The latter was provided with grease-gun lubrication points to ease maintenance.
A distinguishing feature of the locomotive was the high-set N class boiler, which was pitched above the frames to accommodate the inside cylinder assembly, potentially exposing the conjugation mechanism to the elements. To prevent corrosion, Maunsell incorporated a vertical metal cover above the front buffer beam to protect the inside cylinder and valve gear assembly, the main visual difference from the N class. Maunsell and Holcroft anticipated that the standard N class chimney would choke the exhaust blast produced by the three cylinders. To mitigate this, a wider-diameter blastpipe was cast for the locomotive and a new smokebox fabricated to accommodate a wide-diameter cast chimney. The rest of the construction process followed the 1919 N1 specification, and the final three members of the order for 15 N class locomotives were built as intended. The N1 prototype was completed in December 1922 and attached to a standard Maunsell flat-sided Ashford N class tender. It entered service on 24 March 1923 and the boiler pressure was reduced from to for No. 822's fuel consumption to be compared with that of the 2-cylinder N class during trials.Railway Engineer (1923), p. 143
Southern Railway batch The completion of No. 822 coincided with the absorption of the SECR into the newly created Southern Railway in the 1923 Grouping. Maunsell was appointed the CME of the new company and embarked upon a greatly expanded programme of fleet standardisation to replace the inherited "pre-Grouping" designs. In 1924, No. 822 participated in a series of trials to compare the performance of the N, LSWR S15 and LB&SCR K classes when hauling freight trains of 65 loaded wagons between and .Bradley (1974), p. 139 Despite achieving the lowest coal consumption per mile of all the designs tested, the N1 prototype cost more to maintain and displayed poor steaming qualities that affected timekeeping. After the trials, No. 822 remained the sole N1 until 1929, when the Southern Railway ordered a batch of five locomotives (Nos. A876–A880). The intention was to increase the availability of the class to complement the newly introduced U1 class passenger locomotives on gauge-restricted routes.