Pennsylvania classes M1 and M1a

The PRR's first Mountain was created in 1923 by placing the boiler of the Class I1s Decapod on a new frame. After testing the prototype, 175 production locomotives were built by Baldwin and 25 by Lima. In 1930, 50, 25 and 25 locomotives from Baldwin, from Altoona and from Lima followed of the M1a, which had been improved in details. Not only could they pull heavy passenger trains at high speeds, they were also not overwhelmed by freight trains with more than 100 cars and 4,000 short tons in the mountains. 40 M1a were converted to the M1b in 1946 by increasing the boiler pressure by 20 psi and increasing the size of the firebox.