as early as june 1936, the management of pennsylvania railroad decided to build a new passenger locomotive to replace its aging k4s locomotives. they also hoped that the new s1 steam locomotive would have a performance equal to their gg1 electric engine and would be capable of hauling a 1,000-ton passenger train at 100 mph. a conference was held between baldwin locomotive works officials and w. f. kiesel, j. v. b. duer and w. r. elsey for prr, where prr demanded a passenger locomotive to haul 15 standard cars at 100 mph on level track between paoli and chicago. baldwin presented several 4-8-4 and 4-4-4-4 designs made for other railroads. however, prr rejected the 4-8-4 design in favor of a rigid frame duplex and asked baldwin to consider the wheel arrangement 4-4-6-4. in july 1936, prr requested baldwin locomotive works to submit a design for a 4-8-4 engine capable of handling a 2,000-ton train between colehour and harsimus cove.