be improved at high speed by getting the same power from four smaller cylinders with proportionately larger valves. Valve travel in S1's cylinder was 7-1/2 in, the lap was 1-7/8 in, the lead was 5/16 in, exhaust clearance was 0.25 in, valve diameter was 12 in. Construction The S1 was the largest passenger locomotive ever constructed, and the cast steel locomotive bed plate made by General Steel Castings was the largest single-piece casting ever made for a locomotive. In order to negotiate sharper radius curves, S1 was equipped with lateral motion devices made by Alco on its first and third set of drivers, allowing 57.2 mm (2.25 inches) of lateral play on the axles, but these proved to be inadequate. Unlike other experimental duplex engines like PRR's Class Q1 #6130 4-6-4-4, there were no flangeless wheels or blind drivers adopted on S1. In March 1938, a Chicago and North Western class E-4 4-6-4 "Hudson" #4003 was tested by PRR at Altoona. Based on the test results, PRR decided to adopt 84" drivers and a cylinder pressure of 300 psi for the S1. PRR believed that the large diameter drivers could increase the tractive effort without causing undue slipping. In August 1941, PRR VP-Western Region James M. Symes, a senior official who turned against the idea of duplex engine in later years, approved the extension of stall no. 30 of the Crestline roundhouse to accommodate the S1. The stall had a connection at the back because the S1 could only be turned on a wye, but not on the roundhouse's turntable. Timken roller bearings were equipped on the crosshead pins, all engine trucks, and drive axles as well as the tender trucks. Besides, the lightweight reciprocating parts were manufactured by Timken High Dynamic Steel and designed by Timken engineers. To get enough steel between the crank and axle, the back end of each main rod was offset from the crank in the driver, so the big end made a circle while each side rod pin made a circle. Boiler, steaming quality and streamlining The boiler for the S1 was the largest built by the Pennsylvania Railroad; with of direct heating surface and 500 one-inch diameter tubes, the total heating surface area of S1 was ; it was 99.3% as massive as the boiler for Union Pacific's 4000-class "Big Boy" locomotives. In terms of drawbar horsepower, the S1 was 13% more powerful