In the search for a very powerful express locomotive to replace the K4 Pacifics, the PRR stumbled upon the problem of the excessive moving masses. The seven feet driving wheels would have resulted in extremely long coupling rods in a conventional, eight-coupled locomotive. These would not only have transmitted a strong hammer blow to the rails and locomotive, but could also have broken easily. The solution was the design of the duplex locomotives, which combined two separate steam engines on a common frame. To carry the heavy weight of the locomotive, a three-axle bogie was needed in front of and behind the driving axles. This allowed less than half the locomotive's weight to be used as adhesive weight, further increasing the tendency to slip.

Star designer Raymond Loewy provided the locomotive with streamlined Art Deco-style cladding and, including the eight-axle tender, reached a total length of more than 140 feet. Since they no longer fitted on the turntables, the PRR built special track triangles (wyes) for turning. Despite the bogies that could be moved laterally by 2.25 inches, it soon became clear that the S1 had a tendency to derail not only in many curves, but also in the wyes specially built for them. As a result, they could not be used on the entire line between Chicago and Pittsburgh as planned, but only on two-thirds of this line. In addition, if the drivers slipped, it was almost impossible for the engineer to determine which of the two engines was slipping and had to be braked accordingly. These were the main reasons why series production only came about with the T1 with smaller wheels and two-axle bogies.

With regard to the performance of the locomotive, there are different statements, since it was never run at the highest speeds together with dynamometer cars. What is undisputed is that they could haul the required 1,350-ton trains at 100 mph, giving an output of 7,200 hp. Higher speeds were often made to compensate for delays, although there were different observations regarding the maximum speed. Presumably more than 125 mph (201 km/h) were reached several times, but there are also indications in the range between 135 and 145 mph. Allegedly, the PRR once paid a fine when the locomotive reached 156 mph (251 km/h). Irrespective of this, the one-off, which could not be operated economically, was retired in 1946 and scrapped in 1949.