monthly mileage reports from the hagley library indicated that the s1 racked up 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.