The Lehigh Valley Railroad received six 3-cylinder "Mountain" type locomotive from the American Locomotive Company in 1923 and 1924. These locomotives were designated Class S-1 and were assigned road numbers 5000 through 5005. They had 25 x 28 cylinders, 69" drivers, a 200 psi boiler pressure, a tractive effort of 64,700 lbs and a weight of 369,000 pounds. Initially, all six of these new 4-8-2s were used for fast freight, but high maintenance cost soon had them working passenger service.

In 1939, the Lehigh Valley rebuilt all six of the Class S-1s. The three 25 x 28 cylinders were replaced with two 28 x 32 cylinders which reduced the tractive effort from 64,700 lbs to 61,811 lbs. The rebuilt locomotives weighed 370,000 pounds and were designated as Class S-2.

The Lehigh Valley retired the Class S-2s in 1948 and scrapped them shortly after.