In 1923, the PRR finished the design process on a new steam locomotive with the Whyte notation wheel arrangement of 4-8-2, a class sometimes nicknamed the "Mountain." This was the first-ever example of a Mountain on the PRR thus far. The overall design was not an improvement or extension of the railroad's K-series Pacifics as the wheel arrangement and purpose may suggest, but instead a variant of the railroad's I-series 2-10-0 decapods. The two locomotives shared many parts, including connecting rods and a similar boiler, with the main difference being an enlarged combustion chamber for higher-speed operation. A prototype was built by the railroad's Juniata Shops with road number #6699 which underwent almost 3 years of strict Pennsy testing processes, including static axle and horsepower testing and mainline service. The lead designers were J. T. Wallis, Chief of Motive Power and W. F. Kiesel, Jr, Mechanical Engineer. After officials heard that crews and maintenance workers reported favorably, an order for 200 more 4-8-2's of the same design were ordered.
M1 & M1a/bEdit
The 200 locomotives in that order were classed as the M1. The order itself was split between Baldwin Locomotive Works, which was responsible for 175 units, road numbers 6800 through 6974, and Lima, with only 25 units assigned the road numbers 6975 through 6999. These were built to the specifications as follows: 72" diameter drivers, total locomotive weight without tender of 385,000 pounds, 250 psi boiler capacity, 27" by 30" cylinders, and a tractive effort of 64,550 lbf. A later order was also placed in 1930 for 100 M1a class locomotives, which were essentially the same as the M1 class but included Worthington Feedwater Heaters and two air compressors. The PRR divided this order between Baldwin, responsible for 50 units assigned road numbers 6700 through 6749, Lima, responsible for 23 units with road numbers 6777 through 6799, and the Juniata Shops, responsible for 25 locomotives assigned road numbers 6750 through 6774. Thirty-eight M1a class engines were later converted to M1b's. These featured an increased boiler pressure of 270 psi, thus increased speed. This was made possible through the addition of large firebox circulators, which are tubes passing water through the firebox itself, a method used to greatly increase steam creation. Increased tractive effort was also an improvement of this class. After World War II, the M-series locomotives were the subject of various improvement programs similar to the K-series Pacifics. Headlights were relocated from the smokebox front to the top of the smokebox, the steam generators were moved to the front of the engine, the slat pilot was replaced with a cast steel frame, and a step was added to permit servicing of the air compressor at its new location. The M1 series was also known to be the receiver of several different tenders. Each new tender was an improvement in size and capacity, with the largest being the 16-wheel PRR model 210-F-75B tenders, which were nearly equal in length to the locomotive. All were equipped with water scoops for non-stop passenger service, and later tenders featured doghouses once the M1s were readily placed into freight service. In the later years of their service, M1s were downgraded from fast freight service to secondaries as diesels took over this service. Only one M1 survives today; M1b #6755 at the Railroad Museum of Pennsylvania. It was placed on the National Register of Historic Places in 1979.