The first 28 "Mountains" on the Southern Pacific Lines were built by the American Locomotive Company and were delivered during 1923 and 1924. These locomotives were designated Class Mt-1 and assigned road numbers 4300 through 4327. They had 28 x 30 cylinders, 73" drivers, a 210 psi boiler pressure, a tractive effort of 57,510 lbs and weighed 368,000 pounds. Later, in 1924 another six "Mountains" were delivered from ALCO. These six (road numbers 4385 through 4390) were designated Class Mt-2 and were similar to the Class Mt-1s, but had a 225 psi boiler pressure and a tractive effort of 61,620 pounds.

The Sacramento General Shops of the Southern Pacific built 49 "Mountains" between 1925 and 1930. These were built in three batches: road numbers 4385 through 4345 designated Class Mt-3 in 1925 and 1926, road numbers 4346 through 4366 designated class Mt-4 built between 1926 and 1929 and road numbers 4367 through 4376 designated Class Mt-5 in 1929 and 1930. These locomotives were essentially copies of the Class Mt-1s. There are no surviving examples of the Southern Pacific "Mountains".

Class Mt-1/Mt-3/Mt-4/Mt-5 (Locobase 228)

First batch built by Alco-Schenectady in 1923; later batches in SP shops up through 1929. 69 built altogether, with another 6 (Mt-2; see Locobase 1403) built for the El Paso & Southwestern with different running gear; The Espee Mountains were considered to have been a very successful class. Oil-fired engine. Larger tender held 16,152 gal water, 4,692 gal oil.

Class Mt-2 (Locobase 1403)

Data from EP&SW 10- 1924 locomotive diagrams supplied in May 2005 by Allen Stanley from his extensive collection.

Lone batch among Espee Mountains built for the El Paso & Southwestern (but delivered to the SP) with different running gear. Alco works # were 65788-65793.

They were later rebuilt with 28" cylinders and Walschaert valve gear (as in the other batches). Firebox heating surface eventually included 22 sq ft of arch tubes and 94 sq ft in thremic arches. Steam admission came through 14" piston valves.