Pennsylvania class Q2

The Q2 was created after the duplex prototype of the Q1 class with three driving axles at the front and two at the rear. This was only intended for freight transport and the number of driving axles in the first and second chassis groups was reversed compared to the Q1. As with the Q1, all driving axles were mounted in the frame and the drive was divided to avoid too long connecting and coupling rods. 

Since the front chassis group had only two axles, the cylinders here were smaller than in the rear. In contrast to the Q1, the rear cylinders were still in front of their driving axles. Each group had an independent anti-slip control. Additionally, one axle of the trailing truck had a booster. 

Overall, the Q2 is considered the largest non-articulated steam locomotive of all time. In static testing, one of the locomotives achieved an output of almost 8,000 hp. This enabled it to pull 125 loaded freight cars at a speed of 50 mph. On the other hand, maintenance was very complex.

In addition to the prototype, 25 production locomotives were built, making the Q2 the most successful duplex locomotive. However, the PRR fleet also included the class J1 with the 2-10-4 wheel arrangement which, with a nominal output of 5,650 hp, achieved almost the same starting tractive effort and was significantly easier to maintain. In addition, more and more diesel locomotives were added. This resulted in the Q2 being out of service by 1951.