One of the first classes of cab forward locomotives of the Southern Pacific were Mallets with a total of six coupled axles and one leading and one trailing axle each. Their designation was thus MM-2 for “Mallet Mogul”. In order to improve the running characteristics when operating in front of passenger trains, the leading axle was replaced with a bogie in 1914. In the 1920s, they were converted to simple steam expansion, with a larger boiler and superheater. Despite the new wheel arrangement, the designation was now AM-2 for “Articulated Mogul”. They were retired between 1946 and 1948. 