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.