Pennsylvania Railroad class S2
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.

The locomotive proved to be powerful and capable, with reserves of power at speed and reasonable fuel economy.  The turbine drive was easy on the track and allowed more power at the rail. During a test run officially arranged by PRR on 30 March 1945, S2 #6200 towing a dynamometer car was able to pull a 17-car train over a distance of 48 kilometers (level track) at a speed of  between Fort Wayne and Chicago. Engineer Mr. Flaya Cartwright and Fireman Mr. M.E. Brown were assigned to this official test run. E.S. Cox, a British locomotive engineer, once traveled on the footplate and reported that "100mph was maintained and exceeded for 12 consecutive minutes". While economical at speed, the locomotive was highly-uneconomical at lower speed.  The turbine used less steam than conventional locomotives above , but below that the locomotive used too much steam and fuel. The boiler normally operated at , but at low speed the pressure could drop as low as  . The increased fuel usage at low speeds caused the firebox to run hotter, which sometimes caused stay bolts to break.

The locomotive's problems, the rapid decline of PRR's ridership since 1947 and the advantages of the emerging diesel locomotive ensured that #6200 would never be duplicated, but S2 still participated in the 1948 Chicago Railroad Fair with a T1 4-4-4-4 Duplex Steam Locomotive. In August 1949, the locomotive suffered severe turbine damage. Since the maintenance costs rose steadily, the S2 was finally stored in Crestline and Altoona. In 1952, she was finally retired and was scrapped the following year.

Service history
The S2 was assigned to the Fort Wayne Division and based at the Crestline enginehouse. It hauled various prestigious passenger trains serving the New York to Chicago corridor such as The Broadway Limited, The Liberty Limited, The Trail Blazer, The General, The Manhattan Limited and The Golden Arrow on the route between Chicago and Crestline, Ohio (283 miles/ 446 km). S2 also hauled troop trains and was seen towing express freight trains. PRR S2 #6200, as an experimental prototype of a direct-drive steam turbine locomotive, ran 103,000 miles in total before it was completely withdrawn from service in August, 1949 and would soon await the scrapper's torch. The 6200 was eventually scrapped in Conway, Pennsylvania.

In popular culture 
The model train company Lionel made several models of the S2. The first ones, 671 and 2020, were released in 1946 and were discontinued in 1949. The 681 turbine's first production run was in 1950 and 1951. It was similar to both of the original turbine models but the 681 has Lionel's Magne-Traction feature which makes the wheels magnetic. In 1952, The Korean War caused a shortage of magnetic material, so the 671 was reissued as the 671rr (671 rerun). The 681 returned the next year. In 1954 and 1955, another turbine, numbered 682, was released. The 682 was basically a 681 but with valve gear on the wheels and a white stripe painted on both sides.

See also 
 LMS Turbomotive
 Ljungström locomotives

References

External links
 The Duplex and Experimental Steam Engines of the PRR
 Pennsylvania Railroad S2 steam turbine
 "Modern Power For Today's Trains" - A promotional booklet put out by the PRR in 1949 showcasing the railroads latest motive power. 
 "The Keystone" Magazine - Autumn 2012 Volume 45 - Number 3 - The S2 Turbine 686
 S2 6-8-6 Steam Turbine (2014, in Russian) 
 Pennsylvania Railroad patent drawings (January 20, 2012 Classic Trains Magazine) 
 "Presenting a Line of Modern Coal-Burning Steam Locomotives, December 1944" - A promotional booklet put out by the PRR in 1949 showcasing the railroads latest steam locomotives.
 

Steam turbine locomotives
Baldwin locomotives
6-8-6 locomotives
Steam locomotives of the United States
S2
Individual locomotives of the United States
Experimental locomotives
Scrapped locomotives
Unique locomotives
Passenger locomotives
Standard gauge locomotives of the United States