Despite this, the Ps-4s were used in motive power pool service, where they were called in to pull the mainline passenger trains again whenever one of the diesel locomotives was unavailable.

1401 was spared from scrap and was donated to Smithsonian Institution, where it was put on permanent display at the National Museum of American History in Washington, D.C., around late 1961 as the sole survivor of the Southern Railway Ps-4 class, which have been regarded by Smithsonian curator John H. White Jr. as being "among the most celebrated passenger locomotives operated in the United States...." During the 1920s, the Southern Railway's (SOU) roster consisted of smaller Ps-2 class 4-6-2 Pacifics that could not handle the longer and heavier mainline passenger trains between Washington, D.C., and Atlanta.

[4][7] The Ps-4s had a more spacious cab, smaller 73 in (1,778 mm) driving wheels, a slightly shorter boiler, an additional firebox combustion chamber, and a Worthington 3-B feedwater heater mounted on the running board of the fireman's side.

[3][8] These arrangements made the locomotives produce 47,535 lb (21.6 tonnes) of tractive effort, allowing them to pull fourteen passenger cars at 80 miles per hour (130 km/h) on the Piedmont terrains between Washington, D.C., and Atlanta.

[10][11] The SOU officials, impressed with the Ps-4s' excellent performance, ordered 15 more locomotives from ALCO in 1924, while the other five were assigned to the Cincinnati, New Orleans and Texas Pacific Railway (CNO&TP).

[4] The CNO&TP Ps-4s were numbered 6471-6475 and were assigned to pull the Royal Palm, Ponce de Leon, Queen & Crescent Limited, and Florida Sunbeam trains, among others.

[12][13] Harrison's trip had inspired the appearance of the second order of Ps-4s built in the summer of 1926 by ALCO's Richmond Works by having them painted in Virginian green with gold leaf trimming and lettering.

[10] They featured an Elesco feedwater heater mounted on top of the smokebox between the stack and bell instead of under the running boards.

[10] In comparison with the two-axle bogie, 10,000 US gallons (38,000 L) standard USRA tender design used on the first order, the 1926 Ps-4s were equipped with a larger three-axle bogie tender with a water capacity of 14,000 US gallons (53,000 L) designed for long-distance passenger runs and eliminating multiple water stops.

[14] Because of the Harrison Engines' Virginian green and gold paint scheme, they were signified as the First Ladies of the Pacifics around the SOU system.

[25][31] Additionally, the Ps-4s were in motive power pool service, where they were called in to pull mainline passenger trains again whenever one of the diesel locomotives was unavailable.

1401 of the 1926 batch was salvaged from the scrap line in 1953, per advice from railfan Walter H. Thrall and SOU board member W. Graham Claytor Jr., who requested that the locomotive should be donated to the Smithsonian Institution in Washington, D.C., since it was recognized to be one of the eight Ps-4 locomotives hauling the funeral train of U.S. President Franklin Roosevelt in April 1945.

1401 locomotive was cosmetically restored and transported via flatbed truck to the Smithsonian's under construction National Museum of American History, which opened in early 1964.