PRR Q2
Production locomotives followed from the end of 1944, but these were rather different, the lesson that backwards-facing cylinders next to the firebox were a bad idea having been relearned. The production Q2 locomotives were of 4-4-6-4 arrangement; they were the largest non-articulated locomotives ever built and the most powerful locomotives ever static tested, producing 7,987 hp on the PRR's static test plant. 26 of them were built and 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.
Despite 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 With dieselization, they were the obvious first targets to be withdrawn since they were only a little more capable than the conventional J1 class 2-10-4s but with far higher running costs and maintenance loads.
Read more about this topic: Duplex Locomotive