Based on the Ls-1 class Mallets, the Southern had the No. 4050 built in 1926 with simple expansion and a slightly smaller boiler. Since this proved its worth, four Ls-1 were also converted to simple expansion and in 1928 a further eight locomotives were built again with a larger boiler. This was more efficient mainly because of the thermic syphons. They also received larger tenders and now formed the Ls-2 class. They were the Southern's most powerful locomotives and were also used on the almost five percent steep Saluda Grade. 