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The SR Merchant Navy Class was a class of air-smoothed 4-6-2 Pacific steam locomotive designed for the Southern Railway by Oliver Bulleid. The Pacific design was chosen in preference to several others proposed by Bulleid. The first members of the class were constructed during the Second World War, and the last of the 30 locomotives in 1949. Incorporating a number of new developments in British steam locomotive technology, the design of the Packets was among the first to use welding in the construction process; this enabled easier fabrication of components during the austerity of the war and post-war economies. The locomotives featured thermic syphons and Bulleid's controversial, innovative chain-driven valve gear. The class members were named after the Merchant Navy shipping lines involved in the Battle of the Atlantic, and latterly those which used Southampton Docks, an astute publicity masterstroke by the Southern Railway, which operated Southampton Docks during the period. Due to problems with some of the more novel features of Bulleid's design, all members of the class were rebuilt by British Railways during the late 1950s, losing their air-smoothed casings in the process. The Packets operated until the end of Southern steam in July 1967. A third of the class have survived and can be seen on heritage railways throughout Great Britain.