The Southern Railway took a key role in expanding the 660 V DC third rail electrified network begun by the London & South Western Railway. As a result of this, and its smaller operating area, its steam locomotive stock was the smallest of the 'Big Four' companies. For an explanation of numbering and classification, see British Rail locomotive and multiple unit numbering and classification. Background Post-nationalisation British Railways completed construction of the 'West Country' and 'Merchant Navy' locomotive designs but did not build any further orders. It abandoned the 'Leader' class experiments, and Bulleid left the UK to carry forward his unusual locomotive designs in Ireland. Withdrawal Withdrawal of ex-SR locomotives happened mainly towards the end of steam on the Southern Region (in 1967), the pre-Grouping designs having gone before then as electrification spread across the region. Locomotives of SR design With the heavy emphasis on electrification for the London suburban area and the Brighton mainline, there was little need for new steam locomotive designs. The main steam tasks were boat trains (Dover, Folkestone and Newhaven), West of England, Kent services and freight. When designing steam locomotives, the designers had some interesting constraints that dictated where the locomotive could be used. Due to the hangover from SE&CR days, most of the lines in Kent were of fairly light construction and would not take the weight of a modern express locomotive until well into the 1930s. Hence the extensive rebuilding (and new construction) of 4-4-0 designs at a time when other lines were busily building Pacifics or heavy 4-6-0s. The ex-SER lines also had the problem of the narrow Mountfield and Wadhurst tunnels on the Hastings line, requiring locomotive and rolling stock rather narrower than permitted elsewhere. This problem persisted into British Railways days until eventually the tunnels were single tracked, giving clearance for normal stock. Services for west of Southampton and Salisbury had a different set of problems as neither the Southern Railway nor its constituents installed water troughs, thus leading to large tenders with greater water capacity than those fitted to similar locomotives on other railways. New designs were: Richard E. L. Maunsell (1923–1937) Maunsell also rebuilt, modified or continued the new construction of earlier classes LSWR H15 class – Further production LSWR N15 class – Further production LSWR S15 class – Further production LSWR M7 class – One superheated – not repeated LSWR T9 class – Superheated LSWR