Three factors dictated the type of locomotive that could operate on the South Eastern and Chatham Railway (SECR): the heavy passenger train loadings; the poor track quality; and the weak, lightly built bridges.

[7] Richard Maunsell was appointed CME of the SECR in 1913, following the enforced retirement of Harry Wainwright, who had left a range of competent but unspectacular locomotive classes that struggled to cope with the increased train lengths and loadings.

[9] 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).

[8] 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.

[12] 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.

[13] 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.

[14] In January 1915 Maunsell received authority to build six examples, but, as with the N class, production was delayed due to the use of the Ashford works for wartime armaments manufacture.

Ten more locomotives were ordered by the SECR from Ashford works in June 1920, and to speed delivery the construction of frames, cylinders and side tanks was subcontracted to the Royal Arsenal at Woolwich.

However, further severe delays at Ashford caused by the backlog of repair work meant that the boilers had to be supplied by the North British Locomotive Company.

[11] These were dual-fitted with vacuum and Westinghouse (air) brakes for use with the former London, Brighton and South Coast Railway (LBSCR) rolling stock on the Central section.

This group had modified suspension on the bogie and leading axle, in an attempt to address complaints from the crews of rough riding experienced with earlier members of the class.

[17] A further 20 members of the class were ordered in March 1926 (ten each from Ashford and Brighton works), despite strong reservations expressed by the Operating Department concerning "the wisdom or desirability of placing so many large passenger tanks in service".

[21] These were allocated the numbers A610–A629, and work had begun on building the frames and cylinders when the order was cancelled following an accident at Sevenoaks in 1927 involving locomotive No. A800.

[23] At the Southern Railway's January 1925 Locomotive Committee meeting, when it was decided to use outside contractors to build the K class, Maunsell received authority to retain one set of parts at Ashford works to construct a prototype 3-cylinder 2-6-4 tank.

790 was also tested on fast Cannon Street, Redhill and Tonbridge trains during the spring of 1922, although rough riding between the latter two stations meant slower speeds over that part of route on subsequent runs.

[11] The vacuum-braked Brighton batch was run-in on the Portsmouth route in preparation for operating the Redhill–Reading line, the class regularly hauling the daily Birkenhead–Dover through train.

[13] The bracing proved incapable of counteracting the stresses applied to the frames when travelling at speed and caused excessive vibration on the footplate at higher outputs.

[32] The first derailment occurred at Borough Green & Wrotham, near Maidstone on 31 March, when the flanges of the lead coupled wheel mounted the rails at 60 mph (97 km/h).

A800 River Cray at Sevenoaks, Kent, in August 1927, caused by a combination of a surge in the water tanks and the flanges of the locomotive's lead driving wheels mounting the rail at speed due to poor quality track-work.

[39] The locomotive was hauling a Cannon Street to Deal express with a Pullman carriage when the leading driving wheels derailed at 55 mph (89 km/h) over catch points in a cutting.

[20] In the days following the accident, two K and K1 class engines were trialled on the London and North Eastern Railway's (LNER) Great Northern mainline under the supervision of that company's CME, Nigel Gresley,[40] to gain an unbiased review of their riding qualities.

[42] The 1928 accident inquiry did not attach blame to the Southern Railway for track maintenance or locomotive performance issues, and noted that the prototype had run for eight years over the same stretch of line without complaint.

[44] The management of the Southern Railway realised that to have any success in operating the K class tanks on other parts of the network, vast stretches of track would require upgrading.

[3] However, many of the components discarded during the rebuilding process would later be re-used on another 2-6-4 tank locomotive designed to haul heavy freight on short trips: the 3-cylinder W class of 1932.

Rebuilding took place at Ashford, Brighton and Eastleigh railway works between March and December 1928, where the water tanks, rear bogie and coal bunker were removed.

[5] The rear bogies were later used on the SR W class 2-6-4 tank locomotives (the only subsequent use of this wheel arrangement by the Southern Railway, and their use was restricted to freight operations around London).

[52] From 1925, the K and K1 classes were repainted in a darker olive green livery, introduced by Maunsell, with plain white lining and primrose yellow markings.

[56] Sir Nigel Gresley's independent report on the K and K1 classes during the mainline stability trials stated that they were well designed, mechanically reliable and capable of hauling expresses at high speeds on well-maintained track, which meant that they could have been useful additions to the Southern Railway's suburban commuter fleet.

The impending electrification of the Brighton Main Line, scheduled for 1932 also meant that fewer duties suitable for heavy passenger tank locomotives would be available in the 1930s.