The Pennsylvania Railroad's class Q2 comprised one prototype and twenty-five production duplex steam locomotives of 4-4-6-4

wheel arrangement In rail transport, a wheel arrangement or wheel configuration is a system of classifying the way in which wheels are distributed under a locomotive. Several notations exist to describe the wheel assemblies of a locomotive by type, position, and c ...

. They were the largest non-

articulated locomotive An articulated locomotive is a steam locomotive (rarely, an electric locomotive) with one or more engine units that can move independent of the main frame. Articulation allows the operation of locomotives that would otherwise be too large to neg ...

s ever built and the most powerful locomotives ever static tested, producing 7,987 cylinder

horsepower Horsepower (hp) is a unit of measurement of power, or the rate at which work is done, usually in reference to the output of engines or motors. There are many different standards and types of horsepower. Two common definitions used today are t ...

(5,956 kW) on the PRR's static test plant. 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 ."4-Cylinder Engine Has 78% More Power."
''Popular Mechanics'', December 1944, p. 13. These were an improved version of the previous Q1 class, which was a

4-6-4-4 In Whyte notation, a 4-6-4-4 is a railroad steam locomotive that has four leading wheels followed by six coupled driving wheels, a second set of four driving wheels and four trailing wheels. Other equivalent classifications are: UIC classificatio ...

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 Dieselisation (US: dieselization) is the process of equipping vehicles with a diesel engine or diesel engines. It can involve replacing an internal combustion engine powered by petrol (gasoline) fuel with an engine powered by diesel fuel, as o ...

, they were the obvious first targets to be withdrawn since they were only a little more capable than the conventional J1 class 2-10-4s, but with far-higher operating and maintenance costs.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.

References

{{PRR locomotives 4-4-6-4 locomotives Duplex locomotives Experimental locomotives Q2 Steam locomotives of the United States Scrapped locomotives Freight locomotives Standard gauge locomotives of the United States