Norfolk and Western M Class

By the mid-1900s, the Norfolk and Western Railway (N&W) primarily relied on 2-8-0 "Consolidation" type locomotives to pull the company's heavy freight trains.[1] As freight traffic increased, the N&W decided to place orders for larger locomotives with longer boilers.[1][2] While 2-8-2 "Mikado" locomotives were made available in the railroad industry by that time, the N&W opted to order locomotives with the 4-8-0 wheel arrangement over the 2-8-2, since they felt putting the weight of the firebox over the rear driving wheels, as opposed to the trailing wheels, would increase adhesion.[1]

The 4-8-0 design the N&W created, classified as the M class, was destined to be an upgrade to the Railway's W2 class 2-8-0's.[2] The pilot axle would carry the lightweight smokebox, and the rear driving wheels would carry the heavyweight 45-square-foot (4 m²) firebox.[2] The firebox would be extended to the back wall of the cab, leaving no room for decks to be installed—this design was similar to those on camelback locomotives, but the fireboxes on the M class were conventional, and the cabs were placed over them, as opposed to ahead of them, like on the camelbacks.[3] The engineer and fireman would have to sit in separate sides adjacent to the firebox while operating an M class, and the fireman would often have to stand on the tender deck while shoveling coal.[2][3]

The M class locomotives were also equipped with 56-inch (1,422 mm) diameter driving wheels, 21-by-30-inch (533 mm × 762 mm) cylinders, and a boiler pressure of 200 pounds per square inch (1,379 kPa), and the locomotives would be capable of producing 40,163 pounds-force (179 kN) of tractive effort.[2] 75 of the first M class locomotives (Nos. 375-449) were delivered from the American Locomotive Company's (ALCO) Richmond, Virginia works from June 1906 to February 1907, and 50 (Nos. 450-499) were delivered from the Baldwin Locomotive Works of Philadelphia, Pennsylvania from April to August 1906.[1]

The M class would prove to be a mainstay of N&W motive power, working in mainline, transfer, switching, and branch line service. Many members of the class would undergo modifications over the years, such as the addition of higher-capacity tenders, installation of power reverse gear, superheating, and the replacement of the original Stephenson valve gear with that of the Baker type. Popular in service, crews would come to affectionately nickname the locomotives “Mollies,” a moniker which stuck until their retirement in the late 1950s.

Following the initial success of the class Ms, the N&W would order more 4-8-0s of nearly identical design in the form of the M1s in 1907. These differed in their use of Walschaerts valve gear and piston valves in place of slide valves. This design change, however, severely affected the locomotives’ performance, largely due to a poorly-executed design. The valves’ centerlines, being offset from those of the pistons, resulted in excess running gear wear and improper valve timing. Unsurprisingly, as older motive power on the N&W would be phased out, the M1s were among the first to go.

Design shortcomings would plague the subsequent class of 4-8-0s, the M2s, as well. From their arrival in 1910, issues with poor riding qualities and steaming were apparent. Though utilizing a larger boiler than either the Ms or M1s, the M2s possessed a relatively small heating surface area in the firebox, resulting in considerable difficulty in maintaining steam. In spite of the issues (along with subsequent unsuccessful modifications), the M2s lasted in service for considerably longer than the M1s, being among the few larger examples of non-articulated freight power owned by the N&W. The last examples lasted in service until the late 1950s.