The St. Louis - San Francisco Railway bought a total of thirty 4-8-2s from the Baldwin Locomotive Works to be used in passenger service. Fifteen (road numbers 1500 through 1514) were delivered in 1923, five (road numbers 1515 through 1519) came in 1925 and the final ten (road numbers 1520 through 1529) arrived in 1926.

All thirty of these "Mountains" had 28 x 28 cylinders, 69" drivers, a 210 psi boiler pressure, a tractive effort of 56,800 lbs and total locomotive weight as shown in the chart below.

During the late 1930s, the SLSF needed new locomotives, and money was tight, so the West Springfield Shops undertook a project to build a total of 34 4-8-2s from older locomotives. The first eleven of these rebuilds, outshopped in 1936 and 1937, were built from used 2-10-2 parts and were assigned road numbers 4300 through 4310. They had 27 x 30 cylinder, 70" drivers, a boiler pressure of 250 psi, a tractive effort of 66,400 lbs and weighed 431,110 pounds.

Another 23 4-8-2s were built using the boilers from 2-10-2s between 1939 and 1942. These locomotives were assigned road numbers 4400 through 4422 and had 29 x 32 cylinders, 70" drivers, a boiler pressure of 210 psi, a tractive effort of 68,600 lbs and weighed a whopping 449,760 pounds. They were the heaviest Mountains ever built. Locomotives numbers 4400 through 4412 were built as oil burners and numbers 4413 through 4422 burned coal.

There are six surviving SLSF "Mountains".

• 1501 at a city park in Rolla, MO
• 1519 at the Railroad Museum of Northwest Oklahoma in Enid, OK
• 1522 at the Museum of Transportation in St. Louis, MO
• 1526 at the Museum of the Great Plains in Lawton, OK
• 1527 at Spring Hill Park in Mobile, AL
• 1529 at Frisco Park in Amory, MS

Number 1522 was donated to the Museum of Transportation in the early 1950s. It was restored to operating condition by a volunteer group known as the St. Louis Steam Train Association and has run several excursions. Today, it is undergoing a major overhaul and is scheduled to run again in May of 1999.