By William Pearce
The Pennsylvania Railroad (PRR) was founded in 1846 and headquartered in Philadelphia, Pennsylvania. In the first half of the 20th century, PRR was the largest railroad by traffic and revenue in the United States. At one time, PRR was the largest publicly traded corporation in the world, with a budget larger than that of the U.S. government and a workforce of approximately 250,000 people.
In 1937, PRR sought to design a coal-burning steam locomotive that would pull heavy passenger trains for long runs at better than 60 mph (97 km/h). Accomplishing such tasks typically required the use of two engines pulling a single train (double-heading). PRR also hoped that the performance of the new engine would match that of the new electric locomotives just then coming into service. The new steam locomotive would serve as an experimental prototype for the railroad as it worked to modernize its fleet. The new locomotive was designated as the S1 class, and PRR collaborated with the American Locomotive Company, the Baldwin Locomotive Works, and the Lima Locomotive Works in designing and building the engine. The S1 was built in PRR’s Altoona Works in Altoona, Pennsylvania during 1938. The S1 was given the Altoona serial number 4341 and the PRR number 6100.
The PRR S1 was a unique duplex locomotive that utilized a 6-4-4-6 wheel arrangement. A six-wheel leading truck with 36 in (.91 m) wheels was positioned at the front of the engine. A set of four 84 in (2.13 m) drive wheels followed, trailed by another identical set of four drive wheels. A six-wheel trailing truck with 42 in (1.07 m) wheels was positioned at the rear of the engine. What made the S1 a duplex locomotive was its use of two separate pairs of cylinders mounted to a rigid frame. Each cylinder pair drove a set of four drive wheels. The two trucks and four pairs of drive wheels were mounted to a single-piece frame bed made of cast steel by General Steel Castings in St Louis, Missouri. The cylinders and their valve chests were integrally cast with the frame. The frame was 77 ft 9.5 in (23.7 m) long, weighed 97,620 lb (44,280 kg), and was the largest locomotive bed casting ever made. However, the use of a long rigid frame meant that the engine would not be able to operate on tracks with significant curves.
With an overall length of 140 ft 2.5 in (42.7 m), the S1 was the longest rigid frame reciprocating steam passenger locomotive ever built, a fact that earned it the nickname The Big Engine. The S1 was made up of an 81 ft 1.75 in (24.7 m) long engine and a 59 ft .75 in (18.0 m) long tender that carried the locomotive’s coal and water. The engine weighed 608,170 lb (275,862 kg), and its weight was distributed with 135,100 lb (61,280 kg) on the leading truck, 191,630 lb (86,922 kg) on the trailing truck (326,730 lb / 148,202 kg total on the trucks), and 281,440 lb (127,659 kg) on the driving wheels. This distribution meant that less than half (46.28%) of the engine’s weight was on the driving wheels, a configuration that often led to wheel slip.
The tender was supported by two eight-wheel trucks with 36 in (.91 m) wheels. It carried 53,000 lb (24,040 kg) of coal in a front compartment and 24,230 gallons (91,720 L) of water in a rear compartment. When combined with the engine, the 451,840 lb (204,951 kg) tender gave the S1 a total weight of 1,060,010 lb (480,813 kg). The locomotive was 15 ft 6 in (4.7 m) tall and 10 ft 7 in (3.2 m) wide.
An HT type mechanical stoker auger transported coal from the tender to the engine’s firebox. The firebox was 198 in (5.03 m) long and 96 in (2.44 m) wide. Coal was burned in the firebox at around 2,000 °F (1,093 °C). Heat from the firebox flowed through the boiler via 219 tubes that were 2.25 in (57.2 mm) in diameter and 69 flues that were 5.5 in (139.7 mm) in diameter. Each of the tubes and flues was 22 ft (6.7 m) long. The 288 tubes and flues would stretch for 6,336 ft (1,931 m) if laid end to end. The boiler was made from approximately 1 in (254 mm) thick nickel steel. After passing through the tubes, the soot, embers, smoke, and heat from the burning coal flowed into a smokebox at the front of the engine and was subsequently vented into the atmosphere via dual vertical stacks. Spent steam from the cylinders was directed through the smokebox and helped create the draft that drew air into the firebox, through the tubes, and out the stacks. The stacks were approximately 21 in (533 mm) in diameter and protruded 4.875 in (124 mm) above the top of the engine.
The tubes, flues, and firebox of the S1 had a combined evaporative surface area of 5,661 sq ft (525.9 sq m). Heat radiating from these surfaces turned water in the boiler to steam and built up a working pressure of 300 psi (20.7 bar). With a temperature of over 420 °F (215 °C), the wet, saturated steam was collected from slots along the top of a pipe inside the boiler shell. The steam then flowed to the modified Type A superheater, which had a surface area of 2,085 sq ft (193.7 sq m). From the superheater, 69 small superheater elements (tubes) took the wet steam back into the flues. The steam inside the superheater elements was heated well above its saturation value and converted to dry, superheated steam. The superheater elements delivered the dry steam to the steam chamber in the superheater.
The flow of steam in and out of each of the engine’s four cylinders was controlled by a Walschaerts valve gear. A 12 in (305 mm) diameter piston spool valve was mounted in a valve chest above each cylinder. The steam-distribution valve slid back and forth 7.5 inches (191 mm) to allow steam to enter one side of the double-acting cylinder while simultaneously opening the other side to exhaust the previous steam charge. The steam flowed from the center of the valve chest into the front of the cylinder and filled its 9,883 cu in (162 L) volume, pushing the 22 in (558.8 mm) diameter piston back 26 in (660.4 mm) to the rear of the cylinder. The valve then slid rearward to direct steam into the rear part of the cylinder and allow the front part of the cylinder to exhaust. Steam entering the rear part of the cylinder pushed the piston forward to its original position. The cylinder had a smaller return volume of approximately 9,321 cu in (153 L) on account of the 5.25 in (133 mm) diameter piston rod taking up some room. The piston rod extended straight back from the cylinder and was attached to the connecting rod via a crosshead. The connecting rod linked the piston rod to the rear driving wheel in the two-wheel set on each side of the engine. Here, the connecting rod was attached to the coupling rod, which connected the two driving-wheel sets together. The reciprocating parts for each of the four two-wheel driving sets weighed 1,010 lb (458 kg). To aid traction, sand could be deposited on the rails in front of all four front drive wheels and in front of the last pair of rear drive wheels. Two sand boxes were positioned on each side of the engine.
The S1 was designed to haul a 1,200-ton (1,089-t) passenger train at 100 mph (161 km/h). The engine developed around 6,500 indicated hp (4,847 kW) at 100 mph (161 km/h) and had a maximum tractive effort of some 76,400 lbf (339.8 kN) based on an 85% efficiency factor. Without any slip, each rotation of the drive wheels moved the engine 22 ft (6.7 m). At 100 mph (161 km/h), each drive wheel rotated 400 times a minute, and each double-acting piston made 800 strokes. This resulted in roughly 17,781 cu ft (503.5 cu m) of steam passing through the S1’s four cylinders every minute.
The S1 was encased in Art Deco-styled cladding designed by Raymond Loewy. The streamlined cladding consisted of aluminum panels that covered the boiler and extended to a bullet-shaped nose at the front of the engine. Skirt panels covered the lower part of the engine and partially concealed the running gear. The cladding was adorned with chrome handrails and trim accents. The S1’s low-profile stacks were concealed in a fairing atop the engine. Loewy had worked with PRR when he designed the streamlined cladding for the K4 engine 3768 in 1936. Additional K4 engines were streamlined, but not to the extent of 3768. Loewy’s S1 styling was a direct development of his work on engine 3768. It is often claimed that Loewy was awarded US patent 2,128,490 for his S1 design, but this patent was applied for on 17 July 1936 and actually details his work on the K4 engine 3768.
Completed on 31 January 1939, the S1 cost PRR approximately $669,780 USD to build, which is equivalent to $11,912,085 USD in 2018. After undergoing some initial testing, the S1 was showcased at the 1939 World’s Fair held at Flushing Meadows Corona Park on Long Island, New York from 30 April 1939 to 27 October 1940. The entire railroad display was sponsored by 27 railroads from the eastern United States. Still numbered as 6100, the S1 was branded “American Railroads” rather than the “Pennsylvania” it wore later in life. The S1 sat atop a special stand that enabled the locomotive to be operated at speed under its own power. In the stand, the engine’s drive wheels powered generators. Electricity created by the generators was used to power motors that turned the 12 wheels of the leading and trailing trucks and the 16 wheels on the tender. The drive system in the stand was configured so that all wheels turned at the same rpm. While the display was open during the 16-month fair, the S1 was operated daily from 12:00 PM to 8:00 PM at 60 mph (97 km/h). By the end of the fair, the S1 had traveled some 50,000 miles (81,467 km) without moving from the stand.
After the fair, the S1 was finally pressed into service for the PRR in December 1940. While the S1 made for an impressive sight on its special display stand, operating the engine on standard track presented some difficulties. The wide, long, and heavy rigid locomotive could not operate on tracks with tight turns or obstructions, which included most of PRR’s system. PRR sent the S1 to operate on a 283-mile (455-km) straight route of the main line from Chicago, Illinois to Crestline, Ohio. Special facilities were built in Crestline to house and maintain the S1. Even so, the locomotive occasionally derailed during turning operations on a special section of wye track.
In the early 1940s, the S1 was operated in profitable service pulling one of the longest passenger trains for PRR—a 2,000-ton (1,814-t) train consisting of 22 cars. The S1 was popular with crews because of its speed, power, and smooth ride. However, the majority of the S1’s weight rested on the leading and trailing trucks rather than on the engine’s eight drive wheels. Frequent wheel slip was an issue—the engineer needed to be careful opening the throttle, and the duplex engine arrangement made it difficult to quickly detect when the drive wheels were slipping. Wheel slip at speed would quickly damage drive components. Some of the S1’s aerodynamic skirting was removed to ease inspection and maintenance. The discarded skirting also allowed better access to the engine’s 350 grease fittings that needed daily servicing. On a standard 283-mile (455-km) run between Chicago and Crestline, the S1 consumed 48,000 lb (21,772 kg) of coal and 36,000 gallons (136,275 L) of water.
In service, the S1 would regularly top 100 mph (161 km/h). On a test run with 12 loaded cars, Charlie Wappes, assistant road foreman of PRR’s Fort Wayne division, observed the S1’s speedometer needle pegged at the gauge’s 110 mph (177 km/h) maximum. Wappes pulled out his stopwatch and timed the train from the Wanatah, Indiana station to the Hanna, Indiana station. The S1 covered the 6.3-mile (10.1-km) distance in 170 seconds, a time that averages to 133.4 mph (214.7 km/h). Other second-hand reports indicate the S1 traveling over 140 mph (225 km/h) on multiple occasions, and an inconceivable top speed of 156 mph (251 km/h) was claimed on a run between Fort Wayne, Indiana to Chicago, Illinois. PRR was reportedly fined for this speed, as the track’s limit was 80 mph (129 km/h). The official (and still current) speed record for a steam locomotive was set by the British LNER (London and North Eastern Railway) Class A4 4468 Mallard at 125.88 mph (202.58 km/h) on 3 July 1938. While it seems possible that the S1 may have been able to break the record, the S1 never made any official speed record attempts, and there is no official documentation that corroborates these high-speed claims.
The S1 was purely an experimental engine, and its operation was very limited. The locomotive was too long for almost all railway turntables, and its long rigid frame could not take the curves into most railyards. But, the S1’s wheel slip trouble, caused by the majority of the engine’s weight resting on the trucks rather than the drive wheels, was perhaps the engine’s biggest issue. After just a few years of operation, the sole S1 was removed from service. Some sources indicate the S1’s last run was in December 1945, while other sources give the date as May 1946. Regardless, the impressive, powerful, and ultimately unsuccessful S1 engine 6100 was scrapped in 1949. However, some of the lessons learned from the S1 were applied to the last steam locomotives built by the PRR, the 4-4-4-4 engines of the T1 class.
– Loco Profile 24: Pennsylvania Duplexii by Brian Reed (June 1972)
– Pennsy Power (I) by Alvin F. Staufer (1962)
– “High-Capacity Locomotive for Fast Service” Railway Age Vol. 106, No. 25 (24 June 1939)
– “Riding the Gargantua of the Rails” by Roderick M. Grant, Popular Mechanics (December 1941)
Mr. Pearce: Many thanks for a good read! Shocked to think that with the best minds of PRR, Alco and Baldwin applied, they could not ensure a slight majority of weight of the engine would be on the driving wheels. I recall reading about development of the CNR 9000, the first successful road diesel locomotive in North America..On its first trip from Kingston, Ontario to Montreal, less than ideal weight distribution was suspected and refined in a short visit to Montreal Locomotive Works. Best wishes! KB
Thanks for the comment. Since the S1 was an experimental locomotive, perhaps everyone thought the weight distribution would be manageable. PRR’s last steam engine, the T1, had a similar issue that was mostly overcome by more delicate use of the throttle. I’m sure the S1 could have been improved, but it was essentially an experimental showpiece.
The S1 has always fascinated me ever since the day I got to know of her. The fact that larger mass was on carrying wheels than driving would in itself not have been such a big problem, mind the locomotive still had as much mass on drivers as did a conventional 4-8-4 of the time. Only, the S1 had two more carrying axles and their mass. That would be about the same as some 4-8-4 had gone from 12 wheel to 16 wheel tenders – they didn’t become ‘slippery’ by that increase of the locomotive total mass in comparison to drivers mass – acceleration with a heavy passenger train would only be a trifle bit slower from the start. You have to look more closely at the design as a whole and its suitability to PRR’s then quite wobbly track (neglected maintenance during wartime.
On the same issue, wobbly track of the old simple rail-to-ties connection with but nails and a minimum of ballast of in itself questionable uniformity and evenness which made it practically a very inefficient and costly proposition to keep very well aligned (not to speak of perfect alignment) I very much doubt any claims of faster running than 115 – 120 mph – and if one driver (hogger) had really tried and let her go – as technically she could have – it would have put the life of passengers at stake in a most audacious manner for that would have been like calling for a most serious derailment of locomotive and coaches. The impact of lateral forces not only increase in square of the increase of speed but detrimental to-and-fro movements are being initiated and build up in succession by imperfections of the rails that would have been dampened out in one sinus cycle at a more moderate speed.
Thus, although the engine and boiler of the S1 would have been able to rev up to high speeds and provide the power to overcome wind resistance I don’t see any stretch of track suitable to support such speeds anywhere in the USA before modern times. If you may, take a look at the way modern high-speed track is being constructed for TGV and ICE trains for 250 km/h and over – and still these high-tech trains are not running completely smooth!
One ironic saying asks why these trains do run so fast: well, they have to because the seats are so cramped you couldn’t sit in them any longer!
BTW – on a fair basis, namely running on level track, not a precipitous ‘downfall’ section, the steam locomotive that still holds the speed record is the German Reichsbahn 05 class, the engine 05 002, on a straight stretch of the Hamburg – Berlin mainline. The 05 class was a three-cylinder 285 psi (3) 17″ x 26″ simple expansion 4-6-4 with special 2.30m drive wheels (90 1/2 inch) and a very free steam circle with long travel 12″ piston valves – and in contrast to the recklessly overdriven Mallard, 05 002 was not about to dismantle her inner drive but was still accelerating when traffic minister Julius Dorpmüller, seated in the test car leading the consist, upon being told they had just passed 200 km/h (124mph), replied “Enough, enough!” and speed was being eased, run made without any harm to the engine.
I was on the train when on the occasion of 150 years celebration of Austrian railways the lighter DR 18 201 three-cylinder Pacific (228 psi (3) 20 1/2″ x 26″ and 90 1/2 inch drivers) ran a scheduled 100 mph with the excursion train: very smooth, very straight running on the perfect stretch from Gloggnitz to the vicinities of Vienna. Some British steam fans who had stopped the train’s speed exclaimed “The acceleration was !electric!” When being informed about this her usual driver Mr. Rindelhardt only remarked “What? well, but I had to observe the line’s speed limits ..” (since the speed rises in sections from leaving the station)
Fastest run with this engine was 113 mph when in service for the locomotive testing department in Halle in the 1960s.
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I’d like to know what the maximum degree of curvature or minimum radius the S1 was designed to run on.
Hello Douglas – I have not had any luck finding that info. Perhaps someone else will.
What Juniatha wrote about the German (05 002) versus Brtish (Mallard) test essays is correct.
May I add: the British Pacific that was urged to speed up was not named for a Wild Eagle nor a Wild Goose but after a short-winged Wild Duck or that’s what Mallard means. That in a way explains why its inside connecting rod bearings had to give up. (just humor)
The remark about the mass distribution is also correct: simply by adding the number of carrying versus driven wheels all wheel arrangements which have a larger number of idling in relation to driven wheels tend to have more mass on the first group especially as they grow heavier and have to use the full admissible mass on all wheels. They are: the 4-4-2, the 4-6-4, the 6-4-4-6.
If there has ever been a stretch of track worth supporting such speeds as claimed for the S1 and the T1, namely 120 mph and over(!), I doubt it, but ok, I have maybe not enough information. That it would have been reckless to put passengers at risk to surpass the line speed limit of 80 mph by 25 % or over is of no doubt to me. It appears nobody had ever tested the physical behavior of tracking at such speeds while it is clear that it will coincide with completely different behavior patterns than 80 mph. The simple T1 front end outside frame bogie does not look trustworthy to me being capable of dampening out sideways kicks at any three-figure speed! Neither does the one of the S1, yet three-axle trucks are generally and inherently more dampening to agitated sinus runs because of the resistance of the middle axle against turning action. It would not tend to be softer to the rails, though. If they are no better mounted than by simple nails driven into the sleeves .. then: “Good night!” as we say in Germany. One malicious result of sidewards patterns pressed into the permanent way by the heavy locomotive’s proper frequency twisting (nosing) could have been that while the loco stays on track, a truck of the lighter cars might jump the rails and then the whole train seeks its freedom outsides the strict and scheduled path, with usually less than favorable results.
So, all-in-all I have serious reservations regarding the authenticity of any of those claims of over 100 mph ……..
Sara – 05003