Pennsylvania 4-4-4-4, 6-4-4-6, etc. "Duplex Drive" Locomotives of the USA

Roster by Richard Duley

ClassQty.Road NumbersYear BuiltBuilderNotes
  1. Numbers 6110 and 6111 dropped from roster in 1953 and scrapped
  2. Number 5508 dropped from the roster in 1952 and scrapped. The balance were dropped in 1953 and scrapped.
  3. Numbers 5525-5529 dropped from the roster in 1953 and scrapped.
  4. Numbers 5517 and 5522 dropped from the roster in 1952. The balance were dropped in 1952 and scraped.
  5. Numbers 5530-5541 dropped from the roster in 1953. Numbers 5542-5549 were sold for scrap in 1956.

Class Details by Steve Llanso of Sweat House Media

Class Q1 (Locobase 349)

Data from Pennsylvania data card and photo E.13377 reproduced on See an extended, entertaining dialogue among several thoughtful ferro-equinologists on duplex setups in general and the Pennsy engines in particular at on duplex setups at's thread (last accessed on 22 March 2015). Begun by Allen Hazen on 7 August 2007, the discussion ranged over several different US duplex designs and delved in great detail. Thanks to Chris Hohl for the valve gear ID.

Only one Q1 was built in 1942. The duplex-drive idea, which had also smitten the Pennsy in the passenger field, took hold of its freight engine designers, too. The Q1's drivers were tall for a freight engine. Its rear two cylinders were reversed under the cab (rods moving forward) to drive the two axles of the second group on the rigid frame. The cylinders sizes and strokes were unalike, which except for compounds, is so unusual as to be unique. The front pair had 23-in diam pistons with a stroke of 28 inches, the rear pair had 19 1/2-in diam pistons with a shorter 26-in stroke.

After further testing, the Pennsy decided to place the two-coupled axle forward and the three-coupled axle in the rear, and to duplex-drive them with conventionally arranged pistons. This resulted in the Q2, described in Locobase 350.

Class Q2 (Locobase 350)

Data from supplied in May 2005 by Allen Stanley from his extensive Raildata collection. Additional comments from Don Ball, Pennsylvania Railroad 1940s-1950s (Chester, Vermont: Elm Tree Press, 1986), pp. 28-29. See an extended, entertaining dialogue among several thoughtful ferro-equinologists on duplex setups in general and the Pennsy engines in particular at on duplex setups at's thread (last accessed on 22 March 2015). Begun by Allen Hazen on 7 August 2007, the discussion ranged over several different US duplex designs and delved in great detail. Pennsy works numbers were 4527 in August 1944, 4535-4536 in January 1945, 4537-4539 in February, 44540-4543 in March, 4544-4548 in April, 4549-4554 in May, and 4555-4559 in June.

It's difficult to see what the Pennsy gained in designing this duplex-drive locomotive so late in the steam locomotive's day. (See Locobase 349 for the single Q1 trial horse.) It is possible that the lesser weight of each driving set (cylinder, main rod, side rods) allowed smoother running at higher speeds. It is, in most respects, virtually identical in size, power, and weight to the J-class 2-10-4s that Pennsy built according to an earlier C&O design, although the tractive effort was 5.5% higher.

The Q2 was a rigid-wheelbase locomotive that was subdivided into two axle groups: the forward pair of axles were driven by 19 3/4-in diameter cylinders with a stroke of 28 inches. The rear cylinder pair, located between the front axle and the 3-coupled rear group, had 23 3/4-in diam cylinders with a stroke of 29 inches. A trailing truck booster added 15,000 lb to the already high starting tractive effort. The engines were large and attractive with an air-smoothed casing similar to that of the SP's 2-8-8-4. 26 were built, including 6131 and the 25 built in 1945-1946.

The Q2 was credited with having developed 8,000 peak horsepower. In tests, the Q2 reached 7,987 hp at 57.4 mph evaporating 16,600 gal of water per hour and burning 12.5 tons of coal. According to Don Ball, the Q-2 was a powerful machine in actual service as well, but he did not think the power worth the Q2's liabilities:

"Things get interesting, however, when you realize the Q2 is, in effect paying a dead weight penalty of 43,220 pounds for 1000 more indicated horsepower at 57 miles per hour [than that developed by the J1]." He concedes that piston thrusts were lower but at freight-train speeds the J1 wouldn't encounter much stress. Against those modest positives, Ball arraigned the Q2 on several charges:

"The Q2's did not know how to conserve water and in short time enginement were told to plan 'no more than an hour and a half of hard running before needing water'". They also were maintenance hogs: "When all the Q2s developed leaks in the barrel seam of the boilers just ahead and back of the second set of cylinders [probably due to frame stress], extensive work was required to do the caulking of the boilers."

Class S1 (Locobase 347)

Passenger locomotive design run wild was the S1. A single example was built in time for the 1939 New York World's Fair, where it was displayed on a treadmill operated by its own steam. COD price was $669,780.

Unfortunately, this exceptionally long engine and tender combination, styled by Raymond Loewy, was prone to slipping and was much too large for most service. Staufer (1968) 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." Indeed, this engine is regarded as the unofficial speed record-holder for steam by many because of runs she reportedly made pulling 850 to 1000-ton trains at 140-145 mph. Note that this high speed is made pulling a weight of train most other passenger locomotives couldn't get up to 50 mph.

Note the very tall drivers, odd-ball 3-axle leading and trailing trucks, and large tender.

The usual tractive effort figure is given as 71,900 lb at 80% cut-off. The figure given in the table permits easier comparison with other engines, most of which have indicated TEs based on the 85% efficiency factor.

Class T1 (Locobase 348)

Data from tables and diagrams in 1947 Locomotive Cyclopedia, supplemented by PRR Steam Locomotive Diagrams supplied in May 2005 by Allen Stanley from his extensive colleciton. See also an extended, entertaining dialogue among several thoughtful ferro-equinologists on duplex setups in general and the Pennsy engines in particular at on duplex setups at's thread (last accessed on 22 March 2015). Begun by Allen Hazen on 7 August 2007, the discussion ranged over several different US duplex designs and delved in great detail.(Thanks to Chris Hohl for querying various data points.)

Baldwin's works numbers were 62451-62452 in 1940 and 72764-72766 in November 1945; 72767-72768 in December; 72769 in January 1946; 72770-72772 in April; 72773-72776 in May; 72777-72780 in June; 72781-72784 in July; 72785-72788 in August.

The Pennsy's own Juniata shops turned out 5500-5524 simultaneously; their works numbers were 4560-4563 in November 1945, 4564-4568 in December, 4569-4570 in January 1946, 4571-4574 in February, 4575-4577 in March, 4578-4580 in April, 4581-4582 in May, and 4583-4584 in June.

The first two engines for the Pennsy in this arrangement were the 6110-6111, built by Baldwin in 1942. An February 2011 email from Mr Andrew Fields pointed out that the only difference between the two was that the 6111 had a Franklin booster engine on the rear truck that added 13,500 lb to the starting tractive effort.

Most sources including surviving locomotive diagrams prepared by the Pennsy itself give the 1,430 sq ft superheater area shown in the specs, but it's worth noting that the 1947 Locomotive Cyclopedia credited the superheater with 1,680 sq ft (156.05 sq m) on page 213.

Setting the style for the fifty that followed (many felt prematurely) in 1945-1946, the 6101s had Franklin poppet valve gear, Raymond Loewy styling with a chisel nose, disc drivers, and most unusually, a rigid wheelbase of four axles divided into two groups, each of which was driven by a pair of cylinders.

The theory behind the duplex drive held that reducing the masses of reciprocating (e.g., main and side rods) and revolving (e.g., cranks and counterweights) would reduce hammering and raise speed. Tests run at a high boiler pressure and late cut-off showed 6,110 drawbar horsepower at 85 mph. In service, these engines could be fast, powerful, and smooth, pulling 910-ton passenger trains at 100 mph.

According to most sources, the divided drive proved to be a major maintenance headache, however, because no way could be found to stop one or the other driver-cylinder set from slipping, either starting or, more alarmingly, at high speed. This liability proved fatal to the chances for real success for these engines, and they were retired well before the much earlier vintage K-4 Pacifics.

Another source of difficulty was the Type A oscillating-cam system that operated the poppet valves. Train Forums contributor David Stephenson, writing as "feltonhill" in a post dated 21 August 2007 (, described the mechanism: "Oscillating cams took their motion from a gearbox containing what amounted to a pair of miniature Walschaerts valve gears. The valve drive cam moved through a small arc but did not fully rotate 360 degrees. Cutoff was controlled within the gearbox mechanism."

Pointing out that other oscillating-cam systems were installed in accessible places on locomotives, feltonhill reports that the T1's installation was: "...a mechanic's nightmare. Although the drive boxes themselves were considered fairly reliable, their location was about as inaccessible as one can imagine and routine mainenance suffered accordingly. The front box was under the "hood" of shrouding on the pilot deck, behind the aftercooler and between the air pumps ...or the rear engine set, the box was located behind the cylinder saddle and positioned vertically between the frames. Nice place to work. Oh, and just for further complication, it was a mirror image of the front drive box."

5500 was converted to a rotary-cam valve system in July 1948 and 5547 was fitted with Walschaert gear in July 1949. The rotary-cam drive's cam "...operated more like those in IC engines, i.e., it rotated 360 degrees and the valve timing was controlled by the contours machined into the cam." Both conversions are reported to have been more reliable and maintainable.

Nine of the engines later had their cylinder diameters reduced by an inch to 18 3/4" (470 mm).

Stephenson offered a more complex view of the design in the May 2005 Chesapeake and Ohio Historical Magazine (reproduced online at, accessed 24 Oct 2005):.

He says that C & O tests of the T1 in regular passenger service show that:

"They handled trains well, particularly at higher speeds.

"They kept schedule and made up delays on most runs.

"They had no excessive tendency to slip.

'The stall at Waynesboro [a September 12, 1946 event often offered as a primary example of the design's slipping tendency] was caused by overloading."

Stephenson's meticulous, and thankfully clearly written, account of the September 1946 tests nevertheless shows some design weaknesses, particularly in starting trains. And he argues that the duplex solution was conceived to redress a potential problem that never quite materialized. As other designers of 4-8-4s came to grips with counterbalancing and stress issues, they came up with such classics as the UP FEFs (Locobase 284), the Santa Fe 2900s (Locobase 271), the N & W Js (Locobase 274), and ultimately the S-1 Niagaras of the New York Central (Locobase 5582). "Here was a locomotive," Stephenson 's verdict concludes, "that could match the T1 at all but the highest speeds, and do it day-in and day-out without special treatment."

Ultimately, the T1s' early departure from service came not from their own peculiar shortcomings, says Stephenson, but from the triumph of the diesel's "...superior economics ...and their immediate application to PRR's heaviest and most prestigious trains ...In the face of dieselization, the T1 just didn't matter."

Specifications by Steve Llanso of Sweat House Media
Locobase ID349 350 347 348
RailroadPennsylvania (PRR)Pennsylvania (PRR)Pennsylvania (PRR)Pennsylvania (PRR)
Number in Class126152
Road Numbers61306175+61005500-5549
Number Built126152
Valve GearWalschaertWalschaertWalschaertFranklin poppet
Locomotive Length and Weight
Driver Wheelbase26.83'26.37'26.50'25.33'
Engine Wheelbase54.83'53.46'64.33'51.92'
Ratio of driving wheelbase to overall engine wheebase 0.49 0.49 0.41 0.49
Overall Wheelbase (engine & tender)107.83'107.62'123.75'107'
Axle Loading (Maximum Weight per Axle)73700 lbs79780 lbs71680 lbs
Weight on Drivers354700 lbs393000 lbs281450 lbs279910 lbs
Engine Weight593500 lbs619100 lbs608170 lbs502200 lbs
Tender Light Weight412900 lbs430000 lbs451840 lbs442500 lbs
Total Engine and Tender Weight1006400 lbs1049100 lbs1060010 lbs944700 lbs
Tender Water Capacity18000 gals19200 gals24230 gals19200 gals
Tender Fuel Capacity (oil/coal)37.5 tons37.5 tons26.5 tons42.6 tons
Minimum weight of rail (calculated)118 lb/yard131 lb/yard117 lb/yard117 lb/yard
Geometry Relating to Tractive Effort
Driver Diameter77"69"84"80"
Boiler Pressure300 psi300 psi300 psi300 psi
High Pressure Cylinders (dia x stroke)23" x 28"19.75" x 28"22" x 26"19.75" x 26"
Low Pressure Cylinders (dia x stroke)19.5" x 26" (2)23.75" x 29" (2)
Tractive Effort81794 lbs100816 lbs76403 lbs64653 lbs
Factor of Adhesion (Weight on Drivers/Tractive Effort) 4.34 3.90 3.68 4.33
Heating Ability
Firebox Area580 sq. ft725 sq. ft660 sq. ft490 sq. ft
Grate Area121.70 sq. ft121.70 sq. ft132 sq. ft92 sq. ft
Evaporative Heating Surface5518 sq. ft6725 sq. ft5661 sq. ft4209 sq. ft
Superheating Surface2290 sq. ft2930 sq. ft2085 sq. ft1430 sq. ft
Combined Heating Surface7808 sq. ft9655 sq. ft7746 sq. ft5639 sq. ft
Evaporative Heating Surface/Cylinder Volume409.82677.37247.44228.28
Computations Relating to Power Output (More Information)
Robert LeMassena's Power Computation36510365103960027600
Same as above plus superheater percentage47098474635029234500
Same as above but substitute firebox area for grate area224460282750251460183750
Power L149777510875089942367
Power MT1546.931432.921594.781334.76


T-1 (1942 Models)
T-1 (1945-46 Models)


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