Pennsylvania Railroad M1

The Pennsylvania Railroad class M1 was a heavy mixed-traffic steam locomotive of the 4-8-2 Mountain arrangement. A single prototype, number 4700, was built at the Altoona Works in 1923 and tested for three years before 200 more were ordered from Baldwin and Lima, numbered in a solid block from 6800 to 6999. Later orders brought the M1a and the M1b with higher boiler pressure. They had 72-inch drivers and a starting tractive effort of 64,550 pounds-force. Built for passenger and freight work, they spent most of their lives on fast freight, and many ran until steam was withdrawn in 1957. M1b number 6755 is preserved.