Pennsylvania Railroad Class Q2 4-4-6-4: Specs, Roster, History

Published: October 2, 2026

The Pennsylvania Railroad’s Class Q2 was a 4-4-6-4 duplex, twenty-six engines built at Altoona for fast freight. The prototype, 6131, went into service on August 28, 1944. The production engines, 6175–6199, followed in 1945, the last of them, 6199, leaving Altoona Works on June 27. Both sets of drivers faced forward. On the railroad’s own 1948 classification the main engine was rated at 100,800 pounds of tractive effort, a trailing-truck booster added 15,000, and the engine in working order weighed 619,100 pounds.

George H. Drury’s chapter, in the revised Guide to North American Steam Locomotives, calls them the most powerful two-unit duplexes anyone built: about 40,000 pounds heavier than a J1, about 5,000 pounds stronger in tractive effort, and more complicated. They were also thirsty and prone to slip. Every one was scrapped.

Pennsylvania Railroad Q2 4-4-6-4 number 6194 at Crestline, Ohio, in July 1947

Pennsylvania Railroad Q2 6194 at Crestline, Ohio, in July 1947. Number 6194 is one of the twenty-five production 4-4-6-4s, 6175–6199, built at Altoona in 1945. Photographer unknown.

On this page: At a glance · A freight duplex · Two engines, one frame · The 1948 form · Tender 180 F 84 · Twenty-six engines · 7,987 indicated horsepower · Crestline and the lines west · Heavier than a J1

At a glance

Item	Detail
Railroad	Pennsylvania Railroad. Class Q2. On a Pennsylvania class an s means superheat. The 1948 form’s letter for these engines is Q2
Whyte	4-4-6-4 duplex. Two engines on one rigid frame. Both sets of drivers faced forward
Builder	Juniata shops, Altoona. Twenty-six engines, all on the Pennsylvania’s own account
Numbers	Prototype 6131, 1944. Production 6175–6199, twenty-five engines, 1945
In service	6131, August 28, 1944, Western Region. 6175, January 18, 1945. 6199 left Altoona Works on June 27, 1945
Cylinders	Front engine 19¾ × 28 in, two driving axles. Rear engine 23¾ × 29 in, three driving axles. Walschaerts gear. Piston valves 12 in and 14 in
Drivers	69 in. Leading wheels 36 in. Trailing wheels 44 in
Boiler	300 psi. Belpaire firebox
Tractive effort	100,800 lb, plus a 15,000-lb booster. Total 115,800 lb. Factor of adhesion 3.90
Weight	On drivers, 393,000 lb. Engine, working order, 619,100 lb. Empty, 558,100 lb
Tender	Class 180 F 84. Water 19,200 gallons. Coal 75,000 lb (37½ tons). Loaded, 422,000 lb
Engine and tender	1,041,100 lb, the working-order engine plus the loaded tender
Test	6175, Altoona test plant, February 1945: 7,987 indicated horsepower. The last locomotive tested there
Retired	6131 in 1952. 6175–6199 in 1953–1956. All scrapped. Every Q2 was retired before any J1

Card dimensions are Pennsylvania Railroad Form 109-J, May 15, 1948, the Q2 column, and tender class 180 F 84 in the same book. Service dates are Christopher T. Baer’s Pennsylvania Railroad chronology, April 2015 edition. Retirement years follow George H. Drury, Guide to North American Steam Locomotives. Information compiled October 2, 2026.

A freight duplex

A duplex puts two engines on one rigid frame. Each engine has its own cylinders, its own rods, and its own set of drivers. There is no hinge between them, so the locomotive is a single wheelbase from the front truck to the trailing truck. The Pennsylvania wanted that arrangement for a freight engine that could use four cylinders, keep each rod assembly shorter than the machinery of a ten-coupled engine, and still put a very large boiler over five driving axles.

The first try was class Q1, engine 6130, a 4-6-4-4 outshopped at Juniata in May 1942. One set of drivers was turned around under the firebox, and the rear cylinders sat beside the ashpan. The Q1 was not repeated. It lasted on the roster until 1952, the same year as the Q2 prototype, and it remained one engine.

The War Production Board had frozen new designs, so the Pennsylvania’s wartime 2-10-4 was a borrowed Chesapeake & Ohio engine rather than a new Altoona design. Those were the J1s: 125 engines built at Altoona in 1942–44, one hundred of them numbered 6401–6500 and twenty-five numbered 6150–6174. The railroad then canceled an order to Lima for twenty-five more 2-10-4s. On March 27, 1944, the Board approved twenty-five Q2s at Juniata in their place. The production numbers, 6175–6199, begin at the next number after that J1 block.

The prototype took the number after the Q1. Number 6131 was placed in service on the Western Region on August 28, 1944, with simplified Loewy streamlining. Baer’s chronology calls it, on that day, the largest non-articulated locomotive in the world. Both sets of drivers faced forward. That was the correction of the Q1.

Two engines, one frame

The front engine has two driving axles and the smaller cylinders, 19¾ by 28 inches. The rear engine has three driving axles and the larger cylinders, 23¾ by 29 inches. Both pairs of cylinders sit ahead of the drivers they turn. Valve gear is Walschaerts, with piston valves of 12 inches and 14 inches, listed on the form in that order with the two cylinder sizes. These are piston valves. The passenger duplex, the T1, was the poppet-valve engine. A new T1, number 5550, is the one now being built.

The boiler is pressed to 300 pounds. The firebox is Belpaire: a flat wrapper, parallel to the inner sheets, with straight staybolts that meet both sheets at a right angle. The J1 beside it on the same 1948 form has a radial-stay wide firebox, the crown the borrowed Chesapeake & Ohio design brought with it. The Q2 went back to the Pennsylvania firebox. The grate is still the large one, 121.71 square feet, 108 3/16 by 162 inches, within a fraction of the J1’s grate.

Five driving axles carry 393,000 pounds. The factor of adhesion printed for the main engine is 3.90. That is a low figure for an engine asked to start a heavy freight train, and it is the mechanical side of the slip problem. Two engines on one frame can lose their feet one at a time. The first production engine made the point immediately. Baer’s entry for January 18, 1945, the day 6175 was placed in service, says the production engines proved slippery and consumed excessive water. They were assigned, at delivery, to the Eastern Division of the Central Region. The prototype had gone to the Western Region.

The 1948 form

Form 109-J, issued at Philadelphia on May 15, 1948, over the name of H. T. Cover, Chief of Motive Power, cards the Q2 in one column. The figures below are that column. They cover the class as the railroad listed it in 1948, prototype and production engines together. The 12.59 square feet on the fire-area line are the fire area. The grate is the 121.71.

Specification	Form 109-J, May 15, 1948
Class / Whyte	Q2. 4-4-6-4
Wheel diameters	36 in leading, 69 in drivers, 44 in trailing
Cylinders	19¾ × 28 in and 23¾ × 29 in
Valves	Walschaerts. Piston valves 12 in and 14 in. Travel 8 in. Lap 1 11/16 in and 1¾ in. Steam ports 2¼ in wide
Cylinder spread	92 in and 95½ in
Journals	Engine truck 8¼ × 6 13/32 in. Drivers 13 × 13⅜ in. Trailer truck 8 × 11 in
Driver wheelbase	26 ft 4½ in
Engine wheelbase	53 ft 5½ in
Engine and tender wheelbase	107 ft 7½ in
Deck-plate height	85 in. Bottom of the drawbar pocket, 37¾ in
Boiler	300 psi. Belpaire. Minimum internal diameter 95 15/16 in
Tubes and flues	51 tubes, 2¼ in. 277 flues, 3½ in. Length between sheets 250 11/16 in
Superheater units	554, at 1 3/16 in, two units to each flue
Heating surface	Flues 6,000 sq ft. Firebox 725. Superheater 2,720. Total 9,445 sq ft
Fire area / grate	Fire area 12.59 sq ft. Grate 108 3/16 × 162 in, 121.71 sq ft
Weight on engine truck	96,050 lb
Weight on drivers	78,810; 79,780; 77,450; 78,280; 78,680. Total 393,000 lb
Weight on trailer truck	130,050 lb
Engine, working order	619,100 lb. Empty, 558,100 lb
Tractive effort	Main engine 100,800 lb. Booster 15,000. Total 115,800 lb
Factor of adhesion	3.90
Tender class	180 F 84

The five driver weights add to the 393,000 pounds the form prints for all drivers, and those three groups of weight, 96,050, 393,000, and 130,050, add to the 619,100. Flues, firebox, and superheater add to 9,445. The 6,725 square feet sometimes quoted as evaporative surface are the flues and the firebox without the superheater.

Tender 180 F 84

The locomotive sheet names tender class 180 F 84. On the tender pages of the same book that class is the last column of the upper table. It is a different tender from the 180 P 84 in the lower table, which carries more coal and weighs more. The Q2’s tender is the F.

Tender 180 F 84	Form 109-J
Water	19,200 gallons
Coal	75,000 lb, which is 37½ short tons
Frame	Cast steel, tank bottom. Width 10 ft 0 in. Frame length 53 ft 11¼ in
Trucks	Two, class 4F5T1, cast steel. Wheels 36 in. Truck wheelbase 13 ft 9 in. Journals 6½ × 12 in
Length over the tender	55 ft 7⅜ in
Tender wheelbase	39 ft 0 in. Truck centers 25 ft 3 in
Empty / loaded	187,000 lb / 422,000 lb

Class 4F5T1 is two four-axle trucks, and each truck’s wheelbase is 13 feet 9 inches. Engine and tender together, 619,100 pounds plus the loaded tender, come to 1,041,100 pounds. That figure is the sum of the two lines. The form prints the two weights and leaves the addition to the reader. The engine-and-tender wheelbase on the locomotive sheet is 107 feet 7½ inches. On February 23, 1944, a 110-foot turntable replaced a 100-foot table at Conway so the duplexes could be turned.

Twenty-six engines

No.	Built	In service	Retired	Notes
6131	Juniata, 1944	August 28, 1944	1952	Prototype. Western Region. Simplified Loewy streamlining. Both driver-sets faced forward. Scrapped
6175–6199	Juniata, 1945	6175 on January 18, 1945. 6199 left Altoona Works on June 27, 1945	1953–1956	Twenty-five engines, in place of twenty-five further J1s. At delivery, the Eastern Division of the Central Region. All retired before any J1. Scrapped

Locobase 350’s compilation of Juniata works numbers gives 4527 to 6131 and 4535–4559 to 6175–6199. That is twenty-six numbers. The class count on the steam roster is the same twenty-six: one prototype and twenty-five production engines. None was preserved.

7,987 indicated horsepower

In February 1945 number 6175 was put on the Altoona test plant. Baer records an indicated output of 7,987 horsepower, a record for that plant, and he records that 6175 was the last locomotive tested there. His own note on the line is that another account, Hirsimaki’s, dates the test to April 1946. The horsepower figure is the February entry. It belongs to 6175, the first production engine, the same engine that had gone into service on January 18.

The boiler that made that horsepower was also drawn for an engine that was never built. On June 22, 1944, the mechanical department’s proposal for an experimental class V1 turbine specified the same boiler as the Q2, with the power to come from turbines rather than from the four cylinders. The V1 stayed a design. In 1948 Baer’s chronology records that the railroad canceled further steam development, the V1 included, and ordered full dieselization.

Crestline and the lines west

The photograph shows where a production engine could be found two years after the test. Number 6194 is at Crestline, Ohio, in July 1947, under the coaling tower, with the machinery of both engines out in the open. Crestline sits on the line from Pittsburgh toward Chicago. It was one of four terminals the boards named for the class in 1948.

On January 28, 1948, the Pennsylvania board authorized extending eleven stalls at the Crestline roundhouse, six stalls at Fort Wayne, and all of the stalls at 55th Street in Chicago. On February 25 the Pittsburgh, Fort Wayne & Chicago board approved eleven stalls at Crestline, eleven at 55th Street, and six at Fort Wayne. On December 8, 1948, the Pittsburgh, Cincinnati, Chicago & St. Louis board authorized extending forty stalls and thirty-five outside tracks at the St. Clair roundhouse in Columbus. Baer’s chronology records those authorizations. Richard Leonard’s captions put 6175 in Chicago on December 1, 1948, and 6180 at Englewood, in Chicago, the same year.

The career on the road was short even so. Leonard’s note on 6180 is that by 1949 most of the Q2s were in storage, their work taken by conventional steam that diesels had released from other assignments. Storage and roster removal are two dates. The steam roster’s retirement years, from Drury, are 1952 for 6131 and 1953–1956 for 6175–6199.

Heavier than a J1

The engine the Q2 was built against is on the same 1948 form. A J1 2-10-4, in the three columns for the bar-frame engine, the cast-bed engine, and the J1a, weighs 572,640 to 574,730 pounds in working order. Its cylinders are 29 by 34 inches, its boiler is at 270 pounds, its valve gear is Baker, and its main-engine tractive effort is 93,750 pounds, plus the same 15,000-pound booster. The Q2 at 619,100 pounds is 44,370 to 46,460 pounds heavier than those three columns, and 7,050 pounds stronger before the booster is counted. Drury’s rounded contrast, the sentence on the steam roster, is the one to remember with the class: about 40,000 pounds heavier than a J1, and about 5,000 pounds stronger in tractive effort. The form is the tighter measurement. His other point stands beside either pair of numbers. They were also more complicated.

The J1s outlasted them. The 2-10-4s were retired in 1957–59. Every Q2 was off the roster before any J1. The Pennsylvania’s last steam ran officially on November 30, 1957, behind engines of older and simpler classes. The twenty-six duplexes had already gone to scrap. What the railroad kept is on the steam roster.

Sources

Pennsylvania Railroad. Form 109-J, Classification and Description of Locomotives and Tenders. H. T. Cover, Chief of Motive Power. Philadelphia, May 15, 1948. The Q2 column: wheel arrangement, cylinders, valves, boiler, heating surfaces, weights, tractive effort, factor of adhesion, and tender class 180 F 84. The same book’s J1 columns, for the weight and tractive-effort comparison, and the tender table for class 180 F 84: water, coal, trucks, length, and weights. The 180 P 84 column is a different tender.
Baer, Christopher T. A General Chronology of the Pennsylvania Railroad Company. April 2015 edition. The 1944, 1945, and 1948 files: Board approval of twenty-five Q2s, March 27, 1944. Conway’s 110-foot turntable, February 23, 1944. The unbuilt V1 and the Q2 boiler, June 22, 1944. Number 6131 in service, August 28, 1944. Number 6175 in service, January 18, 1945, and the note that the production engines proved slippery and used too much water. The Altoona test of 6175, 7,987 indicated horsepower, February 1945, with Baer’s note that Hirsimaki dates it to April 1946. Number 6199 leaving Altoona Works, June 27, 1945. Roundhouse authorizations at Crestline, Fort Wayne, 55th Street in Chicago, and St. Clair in Columbus, 1948. Cancellation of further steam development, including the V1.
Drury, George H. Guide to North American Steam Locomotives. Revised edition. Kalmbach, 2015. Pp. 267–271. The class grid used on this site’s Pennsylvania steam roster: 6131, built 1944, retired 1952, both driver-sets faced forward. 6175–6199, built at Altoona in 1945 in place of twenty-five further J1s, retired 1953–1956, all of them before any J1. The roster’s summary: about 40,000 pounds heavier than a J1, and about 5,000 pounds stronger in tractive effort.
Llanso, Steve. Locobase 350, on steamlocomotive.com. Juniata works numbers 4527 for 6131 and 4535–4559 for 6175–6199.
Leonard, Richard. Captions for Pennsylvania 6175 and 6180, railarchive.net. Number 6175 in Chicago, December 1, 1948, photographed by Charles E. Winters. Number 6180 at Englewood, Chicago, in 1948. By 1949, most of the class in storage. The cylinder arrangement in the 6175 caption, and the 619,100-pound engine weight, agree with Form 109-J.

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