Pennsylvania Railroad class S1

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Pennsylvania Railroad S1
Industrial designer Raymond Loewy stands on the front of the S1
Type and origin
Power type Steam
Builder PRR Altoona Works
Serial number Altoona 4341
Build date 1939
Total produced 1
 • Whyte 6-4-4-6
 • UIC 3′BB3′
Gauge 4 ft 8 12 in (1,435 mm)
Leading dia. 36 in (914 mm)
Driver dia. 84 in (2,134 mm)
Trailing dia. 42 in (1,067 mm)
Wheelbase Coupled: 26 ft 6 in (8.08 m),
Loco: 64 ft 4 in (19.61 m),
Loco & tender: 123 ft 9 14 in (37.73 m)
Length 140 ft 2 12 in (42.74 m)
Width 10 ft 7 in (3.23 m)
Height 16 ft 6 in (5.03 m)
Adhesive weight 281,440 lb (127,700 kg; 127.7 t) 1st Driver: 73,800 lb (33,475 kilograms; 33 tonnes),
2nd Driver: 73,130 lb (33,171 kilograms; 33 tonnes),
3rd Driver: 66,970 lb (30,377 kilograms; 30 tonnes),
4th Driver 67,460 lb (30,599 kilograms; 31 tonnes)
Loco weight 608,170 lb (275,861 kilograms; 276 tonnes)
Tender weight Empty: 197,020 lb (89,370 kg; 89.37 t);
Loaded: 451,840 lb (205,000 kg; 205.0 t)
Tender type 250 P84 16-wheel tender (two 4-axle trucks)
Fuel type Coal
Fuel capacity 52,900 lb (24,000 kg; 24.00 t)
Water cap 24,230 US gal (91,700 l; 20,180 imp gal)
 • Firegrate area
132 sq ft (12.3 m2)
Boiler pressure 300 lbf/in2 (2.07 MPa)
Heating surface 7,746 sq ft (719.6 m2)
 • Firebox 660 sq ft (61.3 m2)
 • Heating area 2,085 sq ft (193.7 m2)
Cylinders Four
Cylinder size 22 in × 26 in (559 mm × 660 mm)
Valve gear Walschaerts
Performance figures
Maximum speed At least 133.4 mph (215 km/h)
Power output 7,200hp (hauling 1200 tons train at 100 mph (161 km/h))
Tractive effort 76,403 lbf (339.86 kN) (at 85% cut-off)
Factor of adh. 3.68
Operators Pennsylvania Railroad
Numbers 6100
Retired 1946
Scrapped 1949
Disposition Scrapped

The PRR S1 class steam locomotive (nicknamed "The Big Engine") was a single experimental locomotive, the longest and heaviest rigid frame reciprocating steam locomotive ever built.[1] The streamlined Art Deco styled shell of the locomotive was designed by Raymond Loewy.[1][2]

The S1 was the only locomotive ever built with a 6-4-4-6 wheel arrangement. It was a duplex locomotive, meaning that it had two pairs of cylinders, each driving two pairs of driving wheels. Unlike similar-looking articulated locomotive designs, the driven wheelbase of the S1 was rigid. In order to achieve stability at fast passenger train speeds (above 100 mph), articulation was not used. The S1 was completed January 31, 1939 and was numbered 6100.

At 140 ft 2 12 in (42.74 m) overall, engine and tender, the S1 was the longest reciprocating steam locomotive ever; it also had the heaviest tender ( 451840lb/205 tonnes ), highest Tractive Effort (76,403 lbf (339.86 kN)) of passenger steam engine when built and largest drivers (84 inches diameter) ever used on a locomotive with more than three coupled axles. The problem of wheel slippage, along with a wheelbase which is too long for many PRR curves, limited the S1's usefulness. No further S1 models were built as focus shifted to the much smaller but practical T1 class in June 1940.

Construction and historical background[edit]

The S1 under construction at Altoona. The smaller boiler in the photo is for a B6 switcher giving a sense of scale.

As early as June 1937, Pennsylvania Railroad officials decided to build a new passenger locomotive to replace its aging K4s locomotive. The PRR officials also hoped that the new S1 steam locomotive would have performance equal to their GG1 electric locomotive and capable of hauling a 1,000-ton passenger train at 100 MPH. [3] A Conference was held between Baldwin Locomotive Works officials and W.F. Kiesel, J.V.B. Duer and W.R. Elsey for PRR; PRR demands a passenger locomotive to haul 15 standard cars at 100 MPH on level track between Paoli and Chicago; Baldwin presented a number of 4-8-4 and 4-4-4-4 designs made for other railroads, PRR rejected 4-8-4 in favor of rigid frame duplex. PRR also asked Baldwin to consider 4-4-6-4. In the next month, F. W. Hankins requested Baldwin Locomotive Works to submit a design for a 4-8-4 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: a 4-4-4-4 passenger locomotive that could haul 1,200 tons but exceeds prior weight and clearance restrictions, a 4-4-4-6 passenger locomotive that could haul 1,200 tons but also exceeds limits, a 4-8-4 freight locomotive with same weight on drivers as an M1a, which fails to meet requirement for a 2,000 ton train, an articulated 4-6-6-4, PRR prefers 4-4-4-4 and asked Baldwin to consider a passenger version with 80" drivers and a freight version with 72" drivers. The cooperation between PRR and Baldwin, without signing any agreement or contract, to develop a new high speed duplex engine was not gone very smoothly. 10 months after the first conference, PRR ended Baldwin Locomotive Work's consultation on developing high-speed duplex passenger locomotive and assigned work to a consortium of Baldwin Locomotive Works, American Locomotive Company and Lima Locomotive Works under a joint contract to develop the duplex design Class S1.[4],

Duplex Design[edit]

The reasoning for a duplex design which had divided drive was partly to reduce hammerblow on the track. Two sets of drivers with four wheels each could have lighter running gear than a locomotive with all four axles coupled together. Also cylinder efficiency could be improved by getting the same power by four smaller cylinders with proportionately larger valves. Baldwins chief engineer at the time believed that the 8-coupled, 2 cylinder locomotives of the time were at or near practical limits in terms of steam flow as well. Using four cylinders was a way to get around that. Theoretically such a locomotive would be more powerful and efficient than a conventional two cylinder design. On 28th April 1937, PRR Board authorized $300,000 for this experimental high-speed passenger locomotive project, the original design would be a 4-4-4-4 non-articulated duplex; concept being promoted by Ralph P. Johnson of Baldwin on grounds would have lighter reciprocating parts and smaller cylinders than a 4-8-4 of equal size.

PRR formed advisory committee including Johnson, William Winterwood and H. Glaenzer of Baldwin, Dan Ennis of American Locomotive Company and William E. Woodard and Samuel Allen of Lima and headed by T.W. Demarest, General Superintendent of Motive Power of Western Region. On 2nd June 1937, PRR officially announced the development of the “Pennsylvania Type” high-speed passenger locomotive, that became 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 plans were finalized and approved it had evolved into a 6-4-4-6.[5]

Largest Passenger Engine ever built[edit]

The S1 was the largest passenger locomotive ever constructed, with an overall length of 140 feet 2 12 inches (42.74 m). At 77 feet (23.47 m) long and a weight of 97,600 pounds (44,300 kg; 44.3 t), the cast steel locomotive bed plate made by General Steel Castings was the largest single-piece casting ever made for a locomotive.[3][6] In order to negotiate sharper radius curves, S1 was equipped with [Lateral motion device] made by ALCO on its first and third set of drivers, allowed 57.2 mm (2.25 inches) of lateral play on the axles, but these were proved inadequate. Unlike another experimental duplex engine like Class Q1 #6130 ( 4-6-4-4 ), there was no flangeless wheel or blind driver used on S1. A C&NW Class E4 4-6-4 "Hudson" type #4003 was tested by PRR in March 1938 at Altoona [7]. Base on the testing result, PRR decided to adopt 84" drivers and 300 p.s.i. pressure for this new duplex engine. PRR believed that large diameter drivers could increase tractive effort without undue slipping. [3]

Massive Boiler and its steaming quality[edit]

The boiler for the S1 was the largest built by the Pennsylvania Railroad, with 660 square feet (61 m2) of direct heating surface and 500 one-inch diameter tubes and flues, the total heating surface area of S1 was 7,746 sq ft (719.6 m2), which was 99.3% as large as the renowned Union Pacific 4000-class 4-8-8-4 locomotive, The "Big Boy". In terms of Drawbar Horsepower, PRR S1 was 13% more powerful than the "Big Boy", which was 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 crosstube 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 feed water 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 shell of the locomotive was designed by Raymond Loewy,[1][2] a design concept base on his earlier streamlining design for PRR K4s #3768 in 1936, for which he received U.S. Patent No. 2,128,490.

Construction Cost[edit]

The cost of an S1 was $669,780.00, equal to $11,783,641 today.[3] which was about 2 times the cost of a PRR T1 4-4-4-4 (#6111 $310,676). No. 6100 was built at Altoona on Dec.21, 1938 without streamlined casing, on the same day it made first road test with two cars. It run backward to Huntingdon as there was no turntable large enough to turn it, then forward back to Altoona at up to 50 MPH. It was stopped and checked for overhang on all tight curves. Assistant Chief of Motive Power-Locomotive Carleton K. Steins (1891-1973) noted superior riding and steaming qualities. [8]Timken roller bearings were applied to the crosshead pins, all engine truck, driving axles, trailer truck and tender trucks of Class S1. In addition, the lightweight reciprocating parts were made from Timken High Dynamic Steel and designed by Timken engineers. [9]

To get enough steel between the crank and axle, the back end of each main rod was offset 1 18 inches (29 mm) from the crank in the driver, so the big end made a 26-inch (660 mm) circle while each siderod pin made a 28 14-inch (718 mm) circle.

World's Fair display[edit]

The S1 at the New York World's Fair of 1939

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 17-acre (6.9 ha) exhibit, which also included the Baltimore & Ohio's duplex locomotive.[6] To reach the Fair the S1 took a circuitous route over the Long Island Rail Road. Many obstacles had to be temporarily removed and other obstacles were passed at a crawl to reach the fairgrounds. At the Fair the drive wheels operated under the locomotive's own steam power.[3] This was done by placing the S1 on a platform that had rollers under the drive wheels. The driver wheels running continuously on the roller platform at 60 mph (97 km/h) all day long during the Fair.

The New York World's Fair was opened for two seasons, from April to October each year, and was officially closed permanently on October 27, 1940. During the 5 months break between October 1939 and April 1940, PRR S1 #6100 was put back on the system for passenger service and road testing. There is photo evidence showing that S1 was hauling the Manhattan Limited, a named train served as an alternative to the Broadway Limited, in November, 1939. [10]

Popular Mechanics described S1 "Pride of American Railroad" in an articles of their June, 1939 issue.[11] After the World's Fair the S1 was relettered and numbered for the Pennsylvania Railroad fleet. The S1 was used by the PRR for publicity purposes as well and its image was featured in calendars, stamps, advertisements and brochures. The American Bank Note Company issued a series of posters to demonstrate their engraving skill in 1939, they were published by the Eaton paper company as part of an advertising campaign, one of the posters depicts the S1 hauling the Broadway Limited.[12]

Service history[edit]

Detail view of the driving wheels and cylinders; note the rollers upon which the wheels rested while on display

The S1 class locomotive was so large that it could not negotiate the track clearances on most of the lines of the PRR system. S1 began its passenger train services starting from December 1940, run between Chicago and Pittsburgh. Its 1st run out of Fort Wayne Indiana Crew was Mr H.H. Lehman (Fireman), C.J. Wappes (Road Foreman) and Frank Ritcha (Engineer). Due to its gigantic size, It tended to derail at a relatively sharp curve just outside the west of Pittsburgh station, therefore, In its brief service life it was restricted to the main line between Chicago, Illinois and Crestline, Ohio (283 miles/446 km). It was assigned to the Fort Wayne Division and based at the Crestline enginehouse.

The S1 hauled some popular and commercially successful passenger trains such as The General , The Trailblazer and The Golden Arrow on this route.[3] According to the PRR's roster on 26 November 1943, The consist of Train #77 The Trailblazer had 14 rebuilt heavyweight passenger cars with streamlined features in total which including one PB70ER passenger baggage car, nine P70KR coaches, two D70CR and D70DR Diner, one P70GSR coach, and one POC70R coach-observation.[13] There are historical photographic and films evidence showing that S1 #6100 was the preferred engine of this popular, highly profitable (carried 35 times more passengers than Broadway Limited in 1939, Gross Revenue equal to $2,260,000 in 1940 ) and one of the longest and heaviest passenger train (more than 1000 tons) during the heyday of PRR's passenger services in early 40s. 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 which had independent suspensions and roller bearing equipped soaked up the bumps and the surging often experienced with duplex locomotives.

Starting tractive effort calculated in the usual way (85% mean effective pressure) comes out 76,400 lbf (340 kN), but the engine used 70% limited cutoff (presumably to increase port openings at short cutoff) so the railroad claimed a correspondingly lower tractive effort.[jargon]

Alleged speed records[edit]

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 133.4 miles per hour (214.7 km/h) with 12 heavyweight passenger cars in its back. There are other stories of the S1 reaching or exceeding 140 miles per hour (230 km/h). In the German trade press and literature from 1945, there was a report of a record run of the S1, citing railroad officials that a speed of 141 miles per hour (227 km/h) had been reached.

Its high speed capability was such that many have claimed that the S1 exceeded on multiple occasions the 126 mph (203 km/h) record steam locomotive speed set in 1938 by the LNER locomotive Mallard. The locomotive was claimed to have exceeded 156 mph (251 km/h) on the Fort Wayne-Chicago run, as it was reported that the PRR received a fine for the feat.

Design flaws (without Official record and sources)[edit]

For an experimental engine that had basked in the limelight, despite the fact that it was designed and manufactured in collaboration by three largest locomotive manufacturers in America and operated by one of the largest railroad company in the world, her road records were unusually secretive. Any accusation about its performance without sources or documented evidence can only be see as rumors and only can be used as a reference.

The lack of curve compatibility led to the S1 not being used for its intended long distance express service, but it still managed to serve between Chicago and Crestline, Ohio which was 283 miles/446 km long for almost 5 and a half years, made it having the longest serving record amongst all experimental steam engine prototype of the PRR like Q1 4-6-4-4 and S2 turbine 6-8-6 during mid-40s.

The S1 had less than half (47%) its total weight on the driving wheels, but its Factor of adhesion is very close to the much more successful PRR Q2, Santa Fe "Northern" 4-8-4 2900s and the renowned N&W Class J 4-8-4. More than half of its weight was being carried by the massive six-wheel pilot (leading) and trailing trucks with Timken roller bearing equipped. This left the two sets of four duplex driving wheels susceptible to wheel slippage, a dangerous condition for any steam locomotive, although during the 5 1/2 years service history of S1, no serious accident occurred or being recorded which was caused by wheel slippage.

If the powered wheels of a steam locomotive lose their adhesion to the rails and freely spin, the result is over speed damage. If the locomotive's engineer does not detect and rectify this situation at once two things will occur: first, the metal tires that surround the driving wheels will be seriously damaged; second, the piston rods, their bearings and the other equipment associated with the reciprocating motion of the engine will be damaged or destroyed. Wheel slippage causes a loss of friction between the wheels and the rails, which in turn allows the wheels and associated hardware to run excessively fast.

Normally, in a standard locomotive when starting under load from standing start, the engineer would observe the rearmost driving wheels and compare their movement to the movement of the locomotive over the ground. As locomotives grew larger, the engineer would listen to the rhythmic sound of the exhaust (the characteristic “chug-chug”) and compare the rhythm of this sound with the movement of the locomotive over the ground.

If, as they opened the engine’s throttle, the rate of increase of the “chug chug” sound was out of sync with the forward progress of the locomotive, they knew the wheels were slipping. A passenger locomotive with 72-inch-diameter (1,829 mm) drivers would travel nineteen feet (5.79 m) (over six yards or 5.49 meters) per “chug-chug”. It was therefore easy to correlate the rhythmic sound of the exhaust of a starting locomotive with the locomotive’s progress from a standing start. An engineer could thereby avoid applying too much power to a starting locomotive (by opening the steam throttle valve too much, and too quickly) and causing the wheels to slip.

The S1’s duplex engine design meant that two separate engines were concurrently and in sequence exhausting into two smoke adjoined stacks. This made it nearly impossible for a locomotive engineer to distinguish the behavior of an individual engine set, based upon the sound of its exhaust. Unless both sets of engines experienced wheel slip simultaneously, the sound of the normal set would mask the sound of the set that was slipping and over speeding. If the wheels began to slip, they would have no knowledge of this fact until the tires of the duplex engine’s wheels had already sustained damage, or they heard the crashing sounds of a duplex engine tearing itself apart in over speed.

The S1 had a further handicap in the area of slippage. Unlike virtually all other steam locomotives that faced wheel slippage when starting, the S1 (because of the very light weight on the driving wheels) would experience engine wheel slip over the road at operating speed.[citation needed] The adhesion of the wheels to the rails was so light that even minor variations in the roadbed would cause either the forward or rearward duplex engines to slip. This hazard more than any other doomed the operating life of this engine. The loss of adhesion at speed (30–50 mph or 48–80 km/h and above) always did serious mechanical damage to the engine which lost adhesion, and to the locomotive as well.[citation needed]

Mitigation Measures[edit]

In order to increase the adhesion and improve performance, an extra massive sand dome was installed above the boiler by PRR, which was located between the first and second drivers. It ensured the supply of sand for steam sanding and slightly increased the axle load above the first and second set axle. S1 was also partially de-skirted in 1942 to improve the visibility of the reciprocating parts for the crews and better operation.[14] Suspension springs of the pilot truck and trailer truck were fine-tuned to straighten out the overall weight distribution in order 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." [15]

The design flaws of the S1 related to curve compatibility and wheel slip led to only one example being produced. A few month after S1 was put in revenue service, The PRR ordered a shorter design that used four wheel lead and trailing trucks to place more weight on the drivers which was developed for Baldwin, the 4-4-4-4 T1 of 1942. Unlike the S1, the T1 was actually used for long distance express service with such trains as the Broadway Limited and the Manhattan Limited, but the T1 was also plagued by wheel slip and related mechanical failures.

Removal from service[edit]

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. 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 Clement announced that “By May of this year (1948) we expect all of our through passenger trains west of the electrified territory to be dieselized”[16]. S1 #6100 was scrapped in 1949.[3][17] The PRR continued developing the T1 class of 4-4-4-4 duplex locomotives but this locomotive model also met with limited success.

Modern culture[edit]

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

  • The S1 was represented in a 1939 painting by railroad artist Grif Teller that appeared in the Pennsylvania Railroad's picture calendar.
  • The American Bank Note Company issued a series of posters published by the Eaton paper company as part of an advertising campaign in 1939, one of the posters depicts the S1 hauling the Broadway Limited
  • CD Album "Night Train, Classic Railroad Songs, Volume 3" by Various Artists, Published by Rounder in 1998, a painting of S1 was used on the cover of the Album.
  • CD Album "Black Diamond Express to Hell" by Various Artists, Published by Rev. A. W. Nix in 2006, features a perspective drawing of S1 which was used as the backbone of the cover's art design.
  • In the PC game Gadget: Invention, Travel, & Adventure, one of the trains the player travels on is pulled by an S1 modeled locomotive.
  • The Danish pop group 'Laid Back's third album 'Play It Straight', released in 1985, features a three-quarter view of the S1's bow on its cover, albeit in a gaudy pinkish red hue it never wore in service.
  • The S1 appears in the Sandman comic series, book IX.
  • In the anime The Galaxy Railways, the engine for Vega Platoon, Iron Burger, is loosely based on the S1, with the 4-8-8-4 wheel arrangement used for the Union Pacific Big Boy, which was used as the bases for Big One, the engine for Sirius Platoon.
  • The S1 appears in the PC game Grim Fandango as "number nine"—the best vehicle for travel to the eternal rest.
  • Solomon Island Stamps release a set of stamp in 2017 (Neofile #SI-17212a) with S1 and other express steam locomotive from all over the world like the LNER A4 Mallard on it, S1 was placed next to the LNER Mallard.[18]

See also[edit]


  1. ^ a b c "Raymond Loewy-Locomotives". The Avanti. Archived from the original on 2007-09-27. Retrieved 2007-09-02. 
  2. ^ a b "It's a bird! It's a plane! No, it's - an office tool". The Home Forum > Essays. The Christian Science Monitor. September 23, 2004. Retrieved 2007-09-04. 
  3. ^ a b c d e f g "Actual Production Models Shown". Brochures. 2003. Retrieved 2007-09-03. 
  4. ^ "PRR Chronology 1936" (PDF). PRR Chronology. Pennsylvania Technical and Historical Society. April 2015. Retrieved 2018-07-21. 
  5. ^ "PRR Chronology 1937" (PDF). PRR Chronology. Pennsylvania Technical and Historical Society. April 2015. Retrieved 2018-07-21. 
  6. ^ a b Reed, Brian (June 1972). Pennsylvania Duplexii. Loco Profile 24. Windsor, Berkshire: Profile Publications Limited. pp. 267–271. 
  7. ^ "Black Gold - Black Diamonds: The Pennsylvania Railroad & Dieselization" Volume 1 by Eric Hirsimaki, July 1997, Page 85-86
  8. ^ "PRR Chronology 1937" (PDF). PRR Chronology. Pennsylvania Technical and Historical Society. April 2015. Retrieved 2018-07-21. 
  9. ^ Waide Collection of Vintage Railroad Advertisements 1840-1949
  10. ^ Pennsy's “Big Engine” in the snow | Classic Trains Magazine | April 1st 2014 |
  11. ^ Popular Mechanics | Million-Pound Iron Horse is 140 Fleet Long {
  12. ^ The Pennsylvania Railroad S-1 |
  13. ^ "Pennsy Streamliners: The Blue Ribbon Fleet by Joe Welsh, 1999, Page 54
  14. ^ Loco Profile, 24 : Pennsylvania DuplexII | 1972
  15. ^ Pennsylvania "Duplex Drive" Locomotives in the USA,
  16. ^ Railway & Locomotive Historical Society Newsletter - Winter 2007
  17. ^ "PRR Chronology 1945" (PDF). PRR Chronology. Pennsylvania Technical and Historical Society. April 2015. Retrieved 2016-02-20.