had helped to create." Suspected design flaws The lack of curve compatibility led to the S1 not being used for its intended long-distance express service. Although the S1 had less than half (47%) its total weight on the driving wheels, its Factor of adhesion was still very close to the much more successful PRR Q2, Santa Fe "Northern" 4-8-4s the 2900s, and the renowned N&W Class J 4-8-4s. More than half of its weight was being carried by the massive six-wheel pilot (leading) and trailing trucks instead of the drivers. This left the two sets of four duplex driving wheels susceptible to wheel slippage. However, during the years service history of the S1, no serious accident occurred due to wheel slippage. The S1 managed to serve between Chicago and Crestline, Ohio for almost years, made it having the longest-serving record among all experimental steam engine prototypes of the PRR, such as the Q1 4-6-4-4 and S2 turbine 6-8-6. Mitigation measures To increase the adhesion and improve performance, PRR enlarged the sand dome on the S1. It ensured the supply of sand for steam sanding and slightly increased the axle load above the first and second set axle. The S1 was partially de-skirted in 1942 to improve the visibility of the reciprocating parts for the crews and better operation. Suspension springs of the pilot truck and trailer truck were fine-tuned to straighten out the overall weight distribution to achieve better performance. Railway Historian and Author Alvin F. Staufer agrees that she (S1) was oversized and thus unable to visit most roundhouses or handle tight curves, but contends: "She was an excellent steamer and gave trouble-free service." Retirement According to an official report from PRR dated December 1, 1945, which is now stored in The Hagley Museum and Library, No. 6100 was awaiting engine truck repairs at Crestline. It was expected to be returned in service after a few days. This is one of a solid proof that the S1 was still in service at least until December 1945. At the time, at least 13 T1 4-4-4-4s were already put into service. The design flaws of the S1 led to only one example being produced. Before the S1 was officially put in revenue service, the PRR already ordered the T1s. Unlike her duplex sisters, the S1 didn't install any form of Poppet Valve Gear, even though it was proposed to install Franklin