In the early 40s, the PRR knew it would need a bigger engine should the US enter the Second World War. So they took a M1 4-8-2, and replaced the behind trailer with a 4 wheel trailer. This allowed the test subject to better go on lighter rails. But the PRR decided not to go on and the M1a was not even placed in a new class. But it did lead to the creation of the R2 and R3 Northern classes right before and during the war.