Although built for both passenger and freight work, they spent most of their service lives hauling heavy high-speed freight trains. Many PRR men counted the M1 class locomotives as the best steam locomotives the railroad ever owned.
Pennsylvania Railroad M1b
4-8-2 No. 6755
The Pennsylvania Railroad's M1 class was a heavy mixed-traffic Mountain locomotive. No. 6755 was built in 1930 as a M1a locomotive and later converted into class M1b. The M1b had greater tractive effort, thanks to the greater pressure, and more power at speed.