The Mountains of the Southern Pacific were intended for long-range passenger trains, sometimes covering more than 800 miles without changing locomotives. The first 28 MT-1s were built by ALCO in Schenectady in 1923 and 1924. The additional batches built by the SP in Sacramento from 1925 to 1930 and totalling 49 were designated MT-3 through MT-5. The MT-2 class was used for six similar locomotives built for the El Paso & Southwestern.

The use of high-tensile steel made lightweight construction possible, which allowed for a larger boiler without exceeding the weight limits. The moving masses in particular were made lighter to improve running at high speeds. They also received a booster on the trailing axle and a Worthington feed water heater. In addition, some, like the one pictured, were retrofitted with a skyline casing on top of the boiler.

Most of the time they were used in front of long-range passenger trains, but also in front of freight trains. In later years they were used in fast suburban traffic. All were scrapped between 1954 and 1959.