The Pennsylvania Railroad's S2 class was a steam turbine locomotive designed and built in a collaborative effort by Baldwin Locomotive Works and Westinghouse Electric & Manufacturing Company, as an attempt to prolong the dominance of the steam locomotive by adapting technology that had been widely accepted in the marine industry. One was built, #6200, delivered in September 1944. The S2 was the sole example of the 6-8-6 wheel arrangement in the Whyte notation, with a six-wheel leading truck, eight driving wheels, and a six-wheel trailing truck. The S2 used a direct-drive steam turbine provided by the Westinghouse Electric & Manufacturing Company, geared to the center pair of axles with the outer two axles connected by side rods; the fixed gear ratio was 18.5:1. Such design was to prevent energy loss and S2 achieved a mechanical efficiency of 97% which means only 3% of steam energy was lost within the propulsion equipment. The disadvantage of a direct-drive steam turbine was that the turbine could not operate at optimal speeds over the locomotive's entire speed range. The S2 was the largest, heaviest and fastest direct-drive turbine locomotive design ever built.
Design specification
The locomotive was to be a 4-8-4, but wartime restrictions on light steel alloys increased weight until six-wheel leading and trailing trucks were needed; its construction was also delayed by World War II. Construction of the locomotive took place at the Baldwin plant in Eddystone. Two turbines were fitted, one for forward travel and a smaller one for reversing at speeds up to . Superheated steam at a rate of about 2,000 pounds per hour was directed onto the turbine blades through 4 nozzles; thousands of turbine blades transmitted steam energy to the transmission gear. The maximum rotation speed of the forward turbine is 9,000 rpm, developing 6,900 hp; the reverse turbine generates 1,500 hp at 8,300 rpm. A monitoring system ensured that the forward turbine could only be started when the reverse turbine was switched off. Weight of propulsion equipment was 39,000 lbs., or 5.65 lbs/hp. A large boiler with a Belpaire firebox and long combustion chamber was fitted. An automatic lubrication system was installed, connecting the transmission case, where the driving gear wheels were immersed in a lubricant reservoir filled with lubricating oil. Lubricant goes through all lubrication points, including roller bearings on all axles via the filtered pipes with the help of two steam pumps. A Worthington-pattern feedwater heater was fitted for increased efficiency. Twin air pumps for train braking were fitted below the running boards beside the smokebox front, and a large radiator assembly at the nose cooled the compressed air.
The turbine exhaust was piped through a set of four nozzles in the smokebox, providing an even draft for the fire and exiting through a unique quadruple stack. S2 had no smoke deflector when it was delivered from Baldwin to PRR in September 1944, but, soon after it began its service, PRR found out that the locomotive blew heavy smoke at lower speed during operation. So, a pair of small smoke deflectors was acquired and a thin semi-circular metal plate for smoke lifting was filled a few feet behind the smokestack. Apparently, these weren't adequate, so a much-larger pair which look like the "elephant ears" used on New York Central class S1b 4-8-4 Niagara and Union Pacific FEF-3 4-8-4 was added at Altoona Works in December, 1946. Using a smoke deflector was never a tradition of the Pennsylvania Railroad. Prior to S2 #6200, there were only three K4s-installed smoke deflectors between 1939 and 1941, for an experimental purpose: PRR #5038, #3876 and #3878. Their smoke deflectors were all removed after the war, so the steam turbine #6200 was the only Pennsy locomotive with huge "elephant ears" after the war.
The tender of the PRR S2 #6200 was originally made for a class L1s locomotive with the designation 180-F-82. Later, it was assigned as 180-P-75 for class K4s No. 3768; the tender was rebuilt again and received the designation 180-P-85 for S2 6200 use. This large 16-wheel tender was similar to that used on the PRR's other large passenger locomotives, the T1 and S1. In order to reduce the crew's workload, a "Fluid Pressure Control System" () designed by Westinghouse engineer Harry C. May was installed on the locomotive. It controlled starting, running, reversing, power output, and speed, and limited speed in both directions of operation, all in one single control lever which was installed in front of the engineer's seat.