Before Pennsylvania Railroad commissioned Baldwin for a 4-4-4-4 duplex passenger engine (T1) in 1940, Baldwin Locomotive Works already begun the development of designs for fast locomotives with duplex drive since 1938, including a rigid-frame 4-2-2-4 and three-cylinder 4-4-4 for lightweight trains and the preliminary design for a 4-4-4-4 for heavy trains; BLW presented these designs to several railroads, but only the PRR adopted the duplex concept. In Oct 1939, When PRR S1 was still on display in the 1939-40 World's Fair, Baldwin Locomotive Works placed a stock order to build a 4-4-4-4 duplex high-speed passenger locomotive as a demonstrator, with bullet nose streamlining by noted industrial designer Otto Kuhler, but before it could be built, the PRR placed an order for two locomotives of this type in July 1940. This implied that PRR S1 was a showpiece exclusively built for the 1939-40 World's Fair instead of replacing the K4s as the new prime power of the PRR, the PRR T1s was developed during the same period of time when the S1 was under construction or on display in the World Fair.