| Chicago Burlington & Quincy Railroad 2-10-2 Class M-2-A 6158 at Philadelphia, Pennsylvania on an unknown day in November 1920, photograph by the Baldwin Locomotive Works, Chuck Zeiler collection. Number 6158 was built in November 1920 (c/n 54121) and retired in March 1954. The class M-2a was an improvement over the class M-2, specifically the use of light-weight running gear. The weight of the reciprocating parts (the rods) produced stress on the track (known as dynamic augment) on each revolution of the wheels, causing damage to the track and roadbed. As the size of the locomotive increased this became increasingly difficult to counter-balance. The Burlington conducted a series of bridge deflection tests around 1914 with four locomotives. The results showed stress at as little as 5 mph with standard rods, and at 40 mph, the stress was 60 percent greater with standard rods than it was with light-weight rods. As a result, the earlier built M-2 class locomotives were fitted with light-weight parts as they came due for class repairs, and the M-2a locomotives were fitted with disc main drivers for faster operation. |