S1 began its passenger train services starting from December 1940, on the run between Chicago and Pittsburgh. On its first run out of Fort Wayne, Indiana, the crew was led by H.H. Lehman (Fireman), C.J. Wappes (Road Foreman) and Frank Ritcha (Engineer). Due to its gigantic size, S1 had to turn on Chicago, Burlington and Quincy Railroad's wye at Chicago before going eastbound and was unable to go through the 130 lb switch (No.8) just to the west of Pittsburgh Union Station[17] This problem wasn't fixed until 1946 which also prevented the PRR class T1 operating through Pittsburgh).
The S1 was so large that it could not negotiate the track clearances on most of the lines of the PRR system, in its brief service life it was restricted to the main line between Chicago, Illinois and Crestline, Ohio (283 miles/446 km). It was assigned to the Fort Wayne Division and based at the Crestline engine-house. Base on photographic evidence, S1 hauled the Broadway Limited (New York to Chicago) and Liberty Limited (Washington D.C to Chicago) in the first few months of its revenue service, and is then assigned to haul other much popular, heavier and commercially successful passenger trains such as The General, The Trail Blazer and The Golden Arrow on this route.[3]
Monthly mileage reports from the Hagley Library indicated that the S1 racked up 10,388-mile (16,718 km) per month or 20 round trips between Chicago, Illinois and Crestline, Ohio in Aug 1941 which was a very spectacular monthly mileage figure for an experimental engine, compared to K4s 6000 to 8000 miles monthly average mileage. This implied that the PRR had high regard for S1's power and speed. The S1 helped PRR to handle the extreme busy Wartime Traffic until the end of WWII and paid off her high construction cost within one year. Crews liked the S1, partly because of its very smooth ride on speed. The great mass and inertia of the locomotive together with the unique, massive 6-wheel trailing truck soaked up the bumps and the surging often experienced with duplex locomotives.