As mentioned above, the PRR Q1 was an attempt to develop a powerful and dual purpose engine. While it had its issues, its design led to the far more successful freight only Q2. The Q2 had a more conventional duplex 4-4-6-4 wheel arrangement and was probably the most successful PRR design. 25 Q2s were built in 1945/6 but they lasted only until 1951 due to relatively high maintenance cost and the introduction of the diesel engine.
The Q2 is MTH, and the L2 is K-Line.