In 1920, ALCO-Schenectady built two 2-6-6-2 Mallets for the Verde Tunnel & Smelter Railroad. It operated a short network to carry copper ore to the smelters in Jerome and Clarkdale, Arizona. The locomotives carried the numbers 500 and 501 and were sold to the Southern Pacific in 1943 for their wartime needs.
The new owner gave them the numbers 3930 and 3931 and created the class name MM-3, where the letters stood for “Mallet Mogul”. There they were used on branches and as helpers, almost exclusively on the Los Angeles division. Since they were not rebuilt to simple expansion, they were among the last compounds in the SP. 3931 was retired in 1951 and 3930 three years later. 