Despite the interruption caused by the conflict, Urie anticipated that peacetime increases in passenger traffic would necessitate longer trains from London to the south-west of England.

It incorporated features from the H15 class, including eight-wheel double bogie tenders with outside plate frames over the wheels and exposed Walschaerts valve gear.

The double bogie tenders were outwardly similar in appearance to those used on the H15s, although strengthened during construction with extra internal bracing to hold 5,000 imperial gallons (22,700 L) of water.

[9] Maunsell's projected design of express passenger locomotive was not ready for introduction during the summer timetable of 1925, so a third batch of ten N15s was ordered for construction at Eastleigh.

[14] The aim was to create a series of standard freight and passenger locomotives for use throughout Britain, and meant that Clayton was privy to the latest GWR developments in steam design.

As part of a process of fleet standardisation, the Operating Department expressed a desire replace obsolescent locomotives on the Eastern and Central sections with the King Arthur class.

E808–E817) were for the former LBSCR routes of the Central section, where short turntables restricted tender size to the 3,500 imp gal (15,900 L) Ashford variety used on the N class.

[13] When Maunsell was told of the decision to name the locomotives, he replied: "Tell Sir Herbert [Walker] I have no objection, but I warn you, it won't make any difference to the working of the engines".

[25] The Southern Railway's motive power re-organisation following the Grouping of 1923 saw the class allocated to sheds across the network and used on Bournemouth to Oxford cross-country trains.

[26] Operations were expanded to more restricted Central and Eastern section mainlines in 1925, and suitably modified class members hauled commuter and heavy boat trains from London Victoria to Dover Marine and expresses to Brighton.

The entire class came into British Railways ownership in 1948: they could be found in most areas of the Southern Region on medium-length expresses and stopping trains on the ex-LSWR mainline.

[9] Through running of the class into Exeter was stopped in favour of engine changes at Salisbury, and Urie attributed the problem to poor driving technique.

[25] The results indicated that better performance could be obtained by altering the steam circuit, valve travel and draughting arrangements, although the first two recommendations were deemed too costly for immediate implementation by the Locomotive Committee.

This was married to a Lemaître multiple-jet blastpipe and wide-diameter chimney, allowing the locomotive to produce performances akin to the more powerful Lord Nelson class.

[22] The soft exhaust of the Lemaître multiple-jet blastpipe precipitated an adjustment to the smoke deflectors on three converted locomotives, with the tops angled to the vertical in an attempt to improve air-flow along the boiler cladding.

E448–E457) ensured continuous fast running on flat sections of track around London, although their propensity for speed was sometimes compromised over the hilly terrain west of Salisbury.

[15] After investigation, the problems were attributed to poor workmanship during construction as the North British Locomotive Company underquoted production costs to gain the contract.

[42] The second batch of "Eastleigh Arthurs" displaced the ex-K class tanks and ex-LBSCR H2 "Atlantic" 4-4-2 locomotives on the Eastbourne and Bognor Regis routes respectively.

E782 Sir Brian was used on the former Great Northern main line for performance trials against the SECR K and K1 class tanks following the Sevenoaks railway accident in 1927.

[44] The tests were supervised by the London and North Eastern Railway's CME, Sir Nigel Gresley, who commented that the class was unstable at high speeds.

[49] These were set in a triangular formation to reduce visibility of exhaust from the air in response to attacks made by low-flying aircraft on Southern Railway trains.

[51] The Maunsell King Arthur examples were easier to maintain, and the large number of modern Bulleid Pacific and British Railways Standard classes were able to undertake similar duties.

[63] The gilt numerals on the cabside and tender rear were replaced by a cast oval plate with "Southern Railway" around the edge and the number located in the centre.

[63] The Board of Directors disapproved and Bulleid repainted the locomotive in darker malachite green with black and white lining (this would later be applied to his Pacifics).

[65] However, from 1942 to 1946, during the Second World War and its aftermath, members of the class under overhaul were turned out in unlined-black livery as a wartime economy measure, with green-shaded sunshine yellow lettering.

[68] As each member of the class became due for a heavy general overhaul, they were repainted in the new standard British Railways express passenger livery of Brunswick green with orange and black lining from April 1949.

[69] After the poor steaming of the Urie batch was addressed, the class proved popular amongst crews, mechanically reliable and capable of high speeds.

The two Maunsell batches with their streamlined steam passages and better draughting arrangements were superior in performance, and were a popular choice when Bulleid's locomotives were unavailable.

[70] Their use of standard parts considerably eased maintenance, and the fitting of different tender and cab sizes meant few operational restrictions for the class on mainline routes.

[70] The electrification of the Eastern and Central sections and the increasing number of Bulleid Pacifics in service meant the lack of a suitable role for the class under British Railways ownership.