Editing LSWR N15 class (section)

==Design and construction==
{{For|detailed information on numbering variations|#Livery and numbering}}
Trials undertaken in 1914 with the H15&nbsp;class prototype had demonstrated to Urie that the basic design showed considerable speed potential on the [[Southern Railway (Great Britain)#Routes|Western section]] of the LSWR from [[Basingstoke railway station|Basingstoke]] westwards, and could form the basis of a powerful new class of 4-6-0 express passenger locomotive with larger {{convert|6|ft|7|in|m|abbr=on}} driving wheels.<ref name=Nock172 /><ref name=Swift2>Swift (2005), p. 92</ref> The LSWR required such a locomotive, which would need to cope with increasing train loads on this long and arduous route to the [[West Country]].<ref name=Nock172 /> The result was the N15&nbsp;class design, completed by Urie in 1917. It incorporated features from the H15&nbsp;class, including eight-wheel double [[bogie]] [[tender (rail)|tenders]] with outside [[Locomotive frame|plate frames]] over the wheels and exposed [[Walschaerts valve gear]].<ref name=Swift2 /> High running plates along the boiler were retained for ease of oiling and maintenance.<ref name=Bradley41 /><ref name=Swift2 /><ref name=Clarke-49>Clarke (April 2008), p. 49</ref>

Despite the similarities, the N15&nbsp;class represented a refinement of the H15 template. The cylinders were increased in size to {{convert|22|×|28|in|mm|abbr=on}} in diameter, the largest used on a British steam locomotive at that time.<ref name=Bradley41 /> The substantial [[fire-tube boiler|boiler]] design was also different from the parallel version used on the H15, and became the first tapered types to be constructed at Eastleigh Works.<ref name=Bradley41 /> Contrary to boiler construction practices elsewhere where tapering began near the firebox, it was restricted to the front end of the N15's barrel to reduce the diameter of the [[smokebox]], and consequently the weight carried by the front bogie.<ref name=Bradley41 /> The design also featured Urie's design of narrow-diameter "stovepipe" chimney, a large dome cover on top of the boiler, and his [[Superheater#Applications|"Eastleigh" superheater]].<ref name=Bradley42>Bradley (1987), p. 42</ref>

==="Urie N15s"===
[[File:Parkstone Down empty stock between Branksome and Parkstone geograph-2680927-by-Ben-Brooksbank.jpg|thumb|right|Urie N15 30740 'Merlin', near Branksome 1951]]
The N15 design was approved by the LSWR management committee, though the order for construction was postponed until wartime control of raw materials was relaxed.<ref name=Bradley41 /> Government approval was obtained in mid–1918, and Eastleigh Works began to produce the LSWR's first new locomotive class since 1914.<ref name=Bradley41 />

The first locomotives, later known by crewmen as the "Urie N15s", were built in two ten-engine batches by the LSWR's Eastleigh Works between 1918–19 and 1922–23.<ref name=Swift-14>Swift (2005), p. 14</ref> Of the first batch, the prototype, No.&nbsp;736 entered service on 31 August 1918, with four more appearing between September 1918 and April 1919.<ref name=Bradley41 /> They shared a similar profile to Urie's H15&nbsp;class with the use of flat-sided Drummond-style cabs with gently curving roofs. 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 {{convert|5000|impgal|L|sigfig=3}} of water.<ref name=Bradley42 /> 
A shortage of copper delayed completion of Nos.&nbsp;741–745, and the last of the batch emerged from Eastleigh in November 1919.<ref name=Bradley44>Bradley (1987), p. 44</ref> After the running-in of Nos.&nbsp;736–745 and an intensification of the LSWR timetable to the West Country, a second batch of ten was ordered in October 1921.<ref name=Bradley44 /> They entered service over the period June 1922 – March 1923, and were numbered in the series 746–755.<ref name=Bradley44 />

At [[Railways Act 1921|Grouping]] in January 1923, the LSWR became part of the new [[Southern Railway (UK)|Southern Railway]], whose [[chief mechanical engineer]] (CME) was [[Richard Maunsell]]. Maunsell planned to introduce his own designs of express passenger locomotive, one of which was to become the future [[SR Lord Nelson class|Lord Nelson class]].<ref name=Herring>Herring (2000), pp. 110–111</ref> Despite this, there was a short-term need to maintain existing services that required modification and expansion of Urie's N15 design.<ref name=Clarke-49 />

===Maunsell's "Eastleigh Arthurs": Drummond rebuilds===
Maunsell's projected design of express passenger locomotive was not ready for introduction during the summer timetable of 1925, so a third batch of 10 N15s was ordered for construction at Eastleigh. This batch was part of an outstanding LSWR order to rebuild 15 of Drummond's unsuccessful 4-cylinder [[LSWR F13 class|F13]], [[LSWR G14 class|G14]] and [[LSWR P14 class|P14&nbsp;classes]] 4-6-0s into 2–cylinder H15&nbsp;class locomotives.<ref name=Clarke-49 /> Only the five F13s were converted to H15s; the remaining 10 G14 and P14s classes (Nos.&nbsp;448–457, renumbered E448–E457) were rebuilt as N15s, implementing modifications to Urie's original design.<ref name=Bradley99 />

The modifications are attributed to Maunsell's Chief Draughtsman [[James Clayton (engineer)|James Clayton]], who had transferred to [[Ashford railway works]] in 1914 from [[Derby Works]]. They were the result of cooperation between the [[South Eastern and Chatham Railway]] (SECR) and the [[Great Western Railway]] (GWR) when Maunsell was seconded to the Railway Executive Committee during the First World War.<ref name=Bradley99 /> 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. These included streamlined steam passages, long-travel valves, the maximisation of power through reduced cylinder sizes and higher boiler pressure.<ref name=Bradley99 />

Maunsell initiated trials with Urie N15 No.&nbsp;442 in 1924, and proved that better performance could be obtained by altering the steam circuit, valve travel and draughting arrangements.<ref name=Clarke-49 /> As a result, Clayton reduced the N15 cylinder diameter to {{convert|20.5|in|mm}} and replaced the safety valves with Ross pop valves set to {{convert|200|psi|MPa|2|abbr=on}} boiler pressure.<ref name=Bradley101>Bradley (1987), p. 101</ref> The Urie boiler was retained, though the Eastleigh superheater was replaced by a Maunsell type with 10 per cent greater superheating surface area.<ref name=Clarke-49 /> This was supplemented by a larger [[Steam locomotive components|steam chest]] and an increased-diameter chimney casting specially designed for the rebuilds.<ref name="Haresnape54" /> It incorporated a rim and [[Capuchon (chimney)|capuchon]] to control exhaust flow into the atmosphere.<ref name="Haresnape54" /> Valve events (the timing of [[valve gear|valve]] movements with the piston) were also revised to promote efficient steam usage and the wheels were re-balanced to reduce hammerblow.<ref name=Bradley101 /><ref name=Nock1976>Nock (1976), Section "N15 class"</ref>

When rebuilding was complete, only the numbers, smokebox doors with centre tightening handles and the flat-sided cabs remained of the G14 and P14&nbsp;classes.<ref name=Haresnape54>Haresnape (1983), p. 54</ref> The rebuilds retained their distinctive Drummond "watercart" tenders, which were modified with the removal of the complex injector feedwater heating equipment.<ref name=Clarke-49 /> The "watercart" tenders were of {{convert|4300|impgal|L|sigfig=3|abbr=on}} water and {{convert|5.00|LT|t|1|abbr=on}} coal capacity.<ref name=Bradley101 /> The 10 rebuilds became the first members of the King Arthur class upon entering service.<ref name=Bradley99>Bradley (1987), p. 99</ref>

==="Scotch Arthurs"===
As the Drummond G14 and P14 4-6-0s were rebuilt to the N15 specification at Eastleigh, a lack of production capacity due to repair and overhaul meant that Maunsell ordered a further batch of 20 locomotives from the [[North British Locomotive Company]] in 1924.<ref name=Swift-19-25>Swift (2005), pp. 19–25</ref> The company had under-quoted to gain the contract, which meant that production of the batch was rushed.<ref name=Bradley103 /> The necessity to maintain an intensive timetable on the Southern Railway's Western section prompted an increase of the order to 30 locomotives (Nos.&nbsp;E763–E792).<ref name=Bradley103 /> Their construction in [[Glasgow]] would gain them the "Scotch Arthurs" nickname in service.<ref name=Herring /> They were all delivered to the Southern Railway by October 1925, and featured the front-end refinements used on the Drummond rebuilds.<ref name=Clarke-49 />

The North British batch was built to the Southern's new composite [[loading gauge]] and differed from previous batches in having an Ashford-style cab based upon that used on the [[SECR N class|N&nbsp;class]].<ref name=Clarke-49 /> Unlike the Drummond cab retained by Nos.&nbsp;448–457 and E741–E755, the Ashford cab was of an all-steel construction and had a roof that was flush with the cab sides, allowing it to be used on gauge-restricted routes in the east of the network.<ref name=Bradley99 /> It was inspired by the standard cab developed in 1904 by [[Richard Deeley|R. M. Deeley]] for the [[Midland Railway]], and was one of a number of Midland features introduced by Clayton to the SECR and subsequently the Southern Railway.<ref name=Clarke-49 /> The smokebox door was revised to the Ashford pattern, which omitted the use of central tightening handles in favour of clamps around the circumference.<ref name=Bradley101 /> The batch was fitted with the Urie-designed, North British-built {{convert|5000|impgal|L|sigfig=3|abbr=on}} capacity double-bogie tenders.<ref name=Bradley99 />

===Maunsell's "Eastleigh Arthurs": second batch===
[[File:SR King Arthur class, 806 Sir Galeron (CJ Allen, Steel Highway, 1928).jpg|thumb|right|alt=Side-and-front view of a 4-6-0 locomotive on a turntable. A member of the crew is standing next to the front.|N15 No.&nbsp;806 ''Sir Galleron'', a member of the second "Eastleigh Arthur" batch. The locomotive is fitted to a six-wheel N&nbsp;class tender for use on restricted Central section turntables]]
With the "Scotch Arthurs" in service, the Southern Railway had an ample fleet of express passenger locomotives for its Western section routes. As part of a process of fleet standardisation, the Operating Department expressed a desire replace obsolescent locomotives on the [[Southern Railway (Great Britain)#Operations|Eastern]] and [[Southern Railway (Great Britain)#Operations|Central sections]] with the King Arthur class.<ref name=Bradley107 /> In May 1925, a batch of 25 locomotives (Nos.&nbsp;E793–E817) based upon the Scotch Arthurs was ordered for construction at Eastleigh with smaller firebox grates and improved water heating surfaces.<ref name=Clarke-50>Clarke (April 2008),  p. 50</ref> After the first 14 (Nos.&nbsp;E793–E806) were built, it was decided to discontinue construction in favour of Maunsell's new 4-cylinder Lord Nelson class design in June 1926.<ref name=Bradley107 />

The Operating Department intended to equip Nos.&nbsp;E793–E807 with six-wheel, {{convert|4000|impgal|L|sigfig=3|abbr=on}} capacity tenders for use on the former SECR lines of the Eastern section.<ref name="Haresnape60" /> These were to replace Scotch Arthurs Nos.&nbsp;E763–E772 on boat train duties.<ref name=Haresnape60>Haresnape (1983), p. 60</ref> This was because the {{convert|5000|impgal|L|sigfig=3|abbr=on}} tenders attached to Nos.&nbsp;E763–E772 were better suited to the longer routes of the Western section.<ref name=Bradley107 /> The final ten engines (Nos.&nbsp;E808–E817) were for the former LBSCR routes of the Central section, where short [[Turntable (rail)|turntables]] restricted tender size to the {{convert|3500|impgal|L|sigfig=3|abbr=on}} Ashford variety used on the N&nbsp;class.<ref name=Bradley107>Bradley (1987), p. 107</ref> After the order was changed to the Lord Nelson class design, 14 N&nbsp;class tenders were fitted to Nos.&nbsp;E793–E806 for use the Central section.<ref name=Bradley107 /> The high [[draw-gear]] (the link between locomotive and tender) of the N&nbsp;class tenders necessitated modification to the frames beneath the cab.<ref name=Clarke-50 />

===Naming the locomotives===
{{For|class details|List of King Arthur class locomotives}}
When the former Drummond G14 and P14 4-6-0s were rebuilt to Maunsell's N15 specification in February 1925, the Southern Railway decided to give names to all express passenger locomotives. Because of the railway's association with the West of England, the Public Relations Officer, [[John Elliot (railway manager)|John Elliot]] suggested that members of the N15&nbsp;class should be named after characters and places associated with the legend of [[King Arthur]].<ref name=Herring /> When Maunsell was told of the decision to name the locomotives, he replied: "Tell [[Herbert Ashcombe Walker|Sir Herbert [Walker]]] I have no objection, but I warn you, it won't make any difference to the working of the engines".<ref name="Nock-241" /> Walker was the General Manager of the Southern Railway, who had told Elliot that Maunsell's permission was required. The first G14 to be rebuilt, No.&nbsp;E453, was given the first name and christened ''King Arthur''. The Urie locomotives (hitherto referred as N15s rather than King Arthurs) were also given names connected with [[Arthurian legend]] and were referred to as "Urie Arthurs"; the Maunsell batches of N15s were nicknamed the "Eastleigh" and "Scotch Arthurs".<ref name="Nock-241">Nock (British Locomotives vol. 1: 1983), p. 241</ref>