The Pennsylvania Railroad's class Q2 comprised one prototype and twenty-five production duplex steam locomotives of 4-4-6-4 wheel arrangement built between 1944 and 1945.^[1][3][4][2]

Pennsylvania Railroad class Q2
PRR Q2 prototype #6131
Type and origin Power type Steam Builder Altoona Works Build date 1944–45[1] Total produced 26[1]
Specifications Configuration: ​  • Whyte 4-4-6-4[1]  • UIC 2'BC2' Gauge 4 ft 8+1⁄2 in (1,435 mm) standard gauge[1] Leading dia. 36 in (0.914 m)[1] Driver dia. 69 in (1.753 m)[1] Wheelbase 26 ft 4.5 in (8.039 m) (driving) 20 ft 4 in (6.20 m) (rigid) 53 ft 5.5 in (16.294 m) Length 124 ft 7.125 in (37.976 m) (locomotive and tender)[1] Width 11 ft 4 in (3.45 m) (storm windows open)[1] Height 16 ft 5.5 in (5.017 m)[1] Adhesive weight 393,000 lb (178,000 kg; 178 t)[1] Loco weight 619,100 lb (280,800 kg; 280.8 t)[1] Tender weight 430,000 lb (195,000 kg; 195 t)[1] Total weight 1,049,100 lb (475,900 kg; 475.9 t)[1] Tender type PRR class 180F84, 8 axles, with conductor "doghouse" Fuel type Soft coal Fuel capacity 39.86 short tons (36.16 t; 35.59 long tons)[1] Water cap. 19,020 US gal (72,000 L; 15,840 imp gal)[1] Fuel consumption 12.5 t (12.3 long tons; 13.8 short tons) of coal per hour 16,600 US gal (63,000 L; 13,800 imp gal) of water per hour Boiler 106 in (2.692 m) Boiler pressure 300 psi (2.1 MPa) Front cylinder 19+3⁄4 in × 28 in (502 mm × 711 mm) Rear cylinder 23+3⁄4 in × 29 in (603 mm × 737 mm) Valve gear Walschaerts
Performance figures Power output 6,645 hp (4,955 kW),[2] (drawbar) Tractive effort 100,816 lbf (without booster) / 115,816 lbf (with booster) Factor of adh. 3.9
Career Operators Pennsylvania Railroad[1] Numbers 6131, 6175–6199[1] Last run 1951 Disposition All scrapped

They were the largest non-articulated locomotives ever built and the most powerful locomotives ever static tested, producing 7,987 cylinder horsepower (5,956 kW) on the PRR's static test plant^{[clarification needed]}. They were by far the most successful duplex type. The duplex propensity to slip was combated by an automatic slip control mechanism that reduced power to the slipping unit.

The Q2 locomotive was 78% more powerful than the locomotives that PRR had in service at the time, and the company claimed the Q2 could pull 125 freight cars at a speed of 50 mph (80 km/h).^[5] These were an improved version of the previous Q1 class, which was a 4-6-4-4 dual-purpose engine instead of a 4-4-6-4 freight engine.

Despite the overall success, the Q2s were all out of service by 1951. With dieselization, they were the obvious first targets to be withdrawn since they were only a little more capable than the conventional J1 class 2-10-4s, but with far-higher operating and maintenance costs. The final Q2, 6199's power output is 7,987 hp. All have been scrapped and none were preserved.