Pennsylvania Railroad class S1


The PRR S1 class steam locomotive was a single experimental duplex locomotive of the Pennsylvania Railroad. It was designed to demonstrate the advantages of duplex drives espoused by Baldwin Chief Engineer Ralph P. Johnson. It was the longest and heaviest rigid frame reciprocating steam locomotive that was ever built. The streamlined Art Deco styled shell of the locomotive was designed by Raymond Loewy.
The S1 had a unique 6-4-4-6 wheel arrangement, meaning that it had two pairs of cylinders, each driving two pairs of driving wheels. To achieve stability at fast passenger train speeds, articulation was not used. The S1 was completed on January 31, 1939, at Altoona shop, and was numbered 6100.
At overall, engine and tender, the S1 was the longest reciprocating steam locomotive ever; it also had the heaviest tender, highest tractive effort of a passenger steam engine when built and the largest drivers ever used on a locomotive with more than three coupled axles. The problem of wheel slippage, along with a wheelbase that was too long for many of the rail line's curves, limited the S1's usefulness. No further S1 models were built as focus shifted to the much smaller but more practical class T1 in June 1940. Design of the 4-4-4-4 T1 and the 6-4-4-6 S1 occurred concurrently, however, the S1 was the first produced.

Construction and historical background

As early as June 1936, the management of Pennsylvania Railroad decided to build a new passenger locomotive to replace its aging K4s locomotives. They also hoped that the new S1 steam locomotive would have a performance equal to their GG1 electric engine and would be capable of hauling a 1,000-ton passenger train at 100 mph. A conference was held between Baldwin Locomotive Works officials and W. F. Kiesel, J. V. B. Duer and W. R. Elsey for PRR, where PRR demanded a passenger locomotive to haul 15 standard cars at 100 mph on level track between Paoli and Chicago. Baldwin presented several 4-8-4 and 4-4-4-4 designs made for other railroads. However, PRR rejected the 4-8-4 design in favor of a rigid frame duplex and asked Baldwin to consider the wheel arrangement 4-4-6-4. In July 1936, PRR requested Baldwin Locomotive Works to submit a design for a 4-8-4 engine capable of handling a 2,000-ton train between Colehour and Harsimus Cove.
Two months after the conference, Baldwin Locomotive Works officials presented four designs to PRR:

Baldwin's proposals for Pennsylvania Railroad in 1936


  1. a 4-4-4-4 passenger locomotive that could haul 1,200 tons but exceeded existing weight and clearance restrictions
  2. a 4-4-4-6 passenger locomotive that could haul 1,200 tons but also exceeded limits
  3. a 4-8-4 freight locomotive with the same weight on drivers as an M1a, which failed to meet the requirements for a 2,000-ton train
  4. an articulated 4-6-6-4 locomotive

PRR preferred 4-4-4-4 and asked Baldwin to consider a passenger version with 80" drivers and a freight version with 72" drivers. However, the cooperation between PRR and Baldwin, which proceeded without signing any agreement or contract, for the development of the new high-speed duplex engine didn't go smoothly. Ten months after the first conference, PRR ended Baldwin Locomotive Work's consultation and assigned the task to a consortium of Baldwin Locomotive Works, American Locomotive Company and Lima Locomotive Works under a joint contract. T. W. Demarest headed the joint committee, General of Motive Power in PRR's Western Region.

The members of the joint committee


  1. Ralph P. Johnsonn
  2. William Winterwood
  3. H. Glaenzer
  4. Dan Ennis
  5. William E. Woodard
  6. Samuel Allen


On 28th April 1937, PRR's Board authorized $300,000 for this experimental high-speed passenger locomotive project. The design started with a 4-4-4-4 duplex.
On 2nd June 1937, PRR officially announced the development of the “Pennsylvania Type” high-speed passenger locomotive which would become Class S1. After various details were discussed and finalized, it became necessary to make changes that substantially increased the locomotive's weight. By the time the plans were finalized and approved it had evolved into a 6-4-4-6.

Duplex design

The benefits of a duplex design included lighter machinery, shorter cylinder stroke, less wear, lower piston thrust, smaller more efficient cylinders, and a more stable
frame than an articulated underframe, also, no hinged connection had to be maintained. Reduced hammer blows on the track resulting in lower maintenance cost. Two sets of drivers with four wheels each could have lighter running gear than a locomotive with all four axles coupled together, smaller and lighter moving parts ensured less wear and tear. Baldwin's chief engineer believed that the 8-coupled, two-cylinder locomotives of the time were at or near practical limits in terms of steam flow, cylinder efficiency could 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”, the lap was 1-7/8”, the lead was 5/16”, exhaust clearance was 0.25”, valve diameter was 12”.

Construction

The S1 was the largest passenger locomotive ever constructed, 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 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 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 and flues, the total heating surface area of S1 was 7,746 sq ft, it was 99.3% as massive as the boiler for Union Pacific's 4000-class 4-8-8-4 locomotive, the "Big Boy". In terms of drawbar horsepower, the S1 was 13% more powerful than the "Big Boy," 7200 hp and 6345 hp respectively. The large Belpaire firebox met the Pennsy's standards; its heating surface area included that supplied by seven American Arch circulators. Water passed through 5 1/2" horizontal tube met at the centerline with the other cross tube forming the bottom of the 7" vertical tube that sprayed the water up into the steam space above the crown sheet. The lowest set pair of tubes was forward with the side openings of the other six steadily rising toward the back.
A large Worthington 6 SA feedwater heater was fitted with a 7 SA pump to handle the enormous boiler's thirst. The six-wheel leading and trailing trucks were added, as the locomotive was too heavy for four-wheel units. The streamlined Art Deco styled shrouding of the locomotive was designed by Raymond Loewy, a design concept based on his earlier streamlining design for PRR K4s #3768 in 1936, for which he received . Raymond Loewy conducted the wind-tunnel test by using the clay model of S1 at Guggenheim Aeronautical Laboratory for the design of its streamlined shrouding, and It was the same laboratory where he conducted testing of the streamlining of PRR K4s #3768. The design of the smoke lifting plate around the smokestack on S1 was improved based on the wind-tunnel test result from Guggenheim..

Construction costs and testing

The cost of the S1 was $669,780.00, equal to $ today. which was over twice the cost of a PRR T1 4-4-4-4. No. 6100 was completed at Altoona on Dec.21, 1938 without streamlined casing. On the same day, it made its first road test with two cars running backward to Huntingdon and returned to Altoona at speeds up to mph. During the run, it was stopped and checked for overhang on all tight curves. Assistant Chief of Motive Power-Locomotive Carleton K. Steins noted superior riding and steaming qualities. During another pre-service road test, the S1 was clocked at towing 90 freight cars.

1939 World's Fair display

The S1 was displayed at the New York World's Fair of 1939–40 with the lettering "American Railroads" rather than "Pennsylvania Railroad", as 27 eastern railroads had one combined exhibit, which also included the Baltimore & Ohio's duplex locomotive. To reach the Fair the S1 took a circuitous route over the Long Island Rail Road. Many obstacles, like some of the third rail guards, had to be temporarily removed and other obstacles were passed at a crawl to reach the fairgrounds. S1 was towed by smaller freight engines like PRR Ils and took a round-about route to the fair site. She ran up the Bel-Del, interchanged with the Lehigh and Hudson River Railway at Belvidere and NH at Maybrook. The S1 traveled over the Poughkeepsie and Hell Gate Bridge, crossed over the Hudson River and then brought it across on the NY Connecting. On March 13, 1939, the S1, lettered “American Railroads” on the tender, arrived at New York World’s Fair.
At the Fair, the drive wheels operated under the locomotive's steam power ran continuously on the roller platform at 60 mph all day long. Film footage shows that all the wheels on S1, besides the drive wheels, were also placed on rollers powered by electricity; every time S1 started its performance by moving the drive wheels, all the wheels were rolling, including the wheels on the tender's truck.
The New York World's Fair was open for two seasons, from April to October each year, and was officially closed on October 27, 1940. During the five-month break between October 1939 and April 1940, #6100 was put back on the system for passenger service and road testing. There is photo evidence showing that the S1 was hauling the Manhattan Limited, a named train serving as an alternative to the Broadway Limited, in November 1939.
Popular Mechanics described S1 as the "Pride of American Railroad" in an article in their June 1939 issue..The New York World's Fair of 1939 attracted nearly 25 million visitors, S1's first 50,000 service mile was accumulated from this fair's live steam show. After the World's Fair, the S1 was re-lettered for the Pennsylvania Railroad fleet. As one of the most important exhibits of the World's Fair, S1 was used by the PRR for various publicity purposes; her image was featured in calendars, stamps, advertisements, brochures, puzzles, etc. The American Bank Note Company issued a series of posters in 1939, published by the Eaton paper company as part of an advertising campaign. One of the posters depicts the S1 hauling the Broadway Limited.

Service history

S1 began its passenger train services starting from December 1940, on the run between Chicago and Pittsburgh. On its first run out of Fort Wayne, Indiana, the crew was led by H.H. Lehman, C.J. Wappes and Frank Ritcha. Due to its gigantic size, S1 had to turn on Chicago, Burlington and Quincy Railroad's wye at Chicago before going eastbound and was unable to go through the 130 lb switch.
The S1 was so large that it could not negotiate the track clearances on most of the lines of the PRR system, in its brief service life it was restricted to the mainline between Chicago, Illinois and Crestline, Ohio. It was assigned to the Fort Wayne Division and based at the Crestline engine-house. Based on photographic evidence, S1 hauled the Broadway Limited and Liberty Limited in the first few months of its revenue service, and is then assigned to haul other much popular, heavier and commercially successful passenger trains such as The General, The Trail Blazer and The Golden Arrow on this route.
Monthly mileage reports from the Hagley Library indicated that the S1 racked up per month or 20 round trips between Chicago, Illinois and Crestline, Ohio in Aug 1941 which was a very spectacular monthly mileage figure for an experimental engine, compared to K4s 6000 to 8000 miles monthly average mileage. This implied that the PRR had high regard for S1's power and speed. The S1 helped PRR to handle the extreme busy wartime traffic until the end of WWII and paid off her high construction cost within one year. Crews liked the S1, partly because of its very smooth ride on speed. The great mass and inertia of the locomotive together with the unique, massive 6-wheel trailing truck soaked up the bumps and the surging often experienced with duplex locomotives.

Operating performance and alleged speed records

In terms of tractive effort and drawbar horsepower, PRR S1 was the most potent reciprocating steam locomotive ever built for passenger service. Starting tractive effort calculated in the usual way comes out, but the engine used 70.6% limited cutoff, so the railroad claimed a correspondingly lower tractive effort. During a test run between Chicago, Illinois and Crestline, Ohio in December 1940, S1 managed to reach on level track with 1350 tons passenger stock behind her, which was equal to 24 postwar lightweight passenger cars. In this test run, S1 also achieved an average speed of, which was 27% faster than the average schedule speed of the route. Some publications from Germany stated that S1 was able to reach in other road tests during the late 1940s with a lighter load behind her, but PRR never claimed this record. On Apr 19, 1941, during an excursion run organized by the Detroit Railroad Club, S1 reached on the route between Fort Wayne and Chicago.
An article "Riding the Gargantua of the Rails" in the Dec 1941 Popular Mechanics Magazine cites a speed recorded by assistant road foreman Charlie Wappes of the Fort Wayne Division during the S1’s test runs at with 12 heavyweight passenger cars in its back. There are other stories of the S1 reaching or exceeding. In the German trade press and literature from 1945, there was a report of a record run of the S1, citing railroad officials of Interstate Commerce Commission that a speed of was reached when the engine was trying to make up time for a delayed westbound train, the Trail Blazer.
Its high-speed capability was such that many have claimed that the S1 exceeded on multiple occasions the record steam locomotive speed set in 1938 by the LNER locomotive Mallard. The engine was claimed to have exceeded on the Fort Wayne-Chicago run, as it was reported that the PRR received a fine for the feat.
The streamlining designer of the train Raymond Loewy himself wrote in 1979: "On a straight stretch of track without any curves for miles; I waited for the S1 to pass through at full speed. I stood on the platform and saw it coming from the distance at 120 miles per hour. It flashed by like a steel thunderbolt, the ground shaking under me, in a blast of air that almost sucked me into its whirlwind. Approximately a million pounds of locomotive were crashing through near me. I felt shaken and overwhelmed by an unforgettable feeling of power, by a sense of pride at the sight of what I 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 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 and trailing trucks instead of the drivers. This left the two sets of four duplex driving wheels susceptible to wheel slippage. However, during the 5 1/2 years service history of S1, no serious accident occurred due to wheel slippage. The S1 managed to serve between Chicago and Crestline, Ohio for almost 5 1/2 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 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, S1 #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 S1 was still in service at least until December 1945. At the time, at least 13 T1 4-4-4-4 were already put into service.
The design flaws of the S1 led to only one example being produced. Before S1 was officially put in revenue service, The PRR already ordered a shorter design, the T1 4-4-4-4 T1 of 1942. Unlike her duplexes sisters, PRR S1 didn't install any Poppet Valve Gear, even though it was proposed to install Franklin Type A rotary cam poppet valve gear when she was already under construction, but such proposal was rejected due to technical difficulties in 1938., this decision unexpectedly prevented a lot of troubles that would have been caused by the problematic Franklin poppet valve gears in S1's service life. The total service years or total service mileage of S1 was ironically slightly longer than some of her younger duplex sisters.
A time-book belonging to Pennsy engineman Byron Breininger from the Ft. Wayne division records a trip to Chicago on S1 engine #6100 at 8:59 AM on May 5, 1946, this run was possibly one of its last in service. Less than two years later, PRR president Martin W. Clement announced that “By May of this year we expect all of our through passenger trains west of the electrified territory to be dieselized”. Preservation of S1 was discussed inside PRR's board, but due to the deteriorating financial situation since 1946, S1 #6100 was scrapped in 1949. The PRR continued developing the T1 class of 4-4-4-4 duplex locomotives but wheel slip and mechanical failures also plagued the T1.

Historical facts of why the PRR T1 wasn't the successor of the S1


Before Pennsylvania Railroad commissioned Baldwin for a 4-4-4-4 duplex passenger engine in 1940, Baldwin Locomotive Works already begun the development of designs for fast locomotives with duplex drive since 1938, including a rigid-frame 4-2-2-4 and three-cylinder 4-4-4 for lightweight trains and the preliminary design for a 4-4-4-4 for heavy trains; BLW presented these designs to several railroads, but only the PRR adopted the duplex concept. In Oct 1939, When PRR S1 was still on display in the 1939-40 World's Fair, Baldwin Locomotive Works placed a stock order to build a 4-4-4-4 duplex high-speed passenger locomotive as a demonstrator, with bullet nose streamlining by noted industrial designer Otto Kuhler, but before it could be built, the PRR placed an order for two locomotives of this type in July 1940. This implied that PRR S1 was a showpiece exclusively built for the 1939-40 World's Fair instead of replacing the K4s as the new prime power of the PRR, the PRR T1s was developed during the same period of time when the S1 was under construction or on display in the World Fair.

Cultural references

The stylish appearance of the S1 has proved to be very popular: