Chicago Burlington & Quincy Railroad 2-10-2 Class M-2-A 6109 presumed at Philadelphia, Pennsylvania on an unknown day in June 1914, photograph by the Baldwin Locomotive Works, print by H. L. Broadbelt, Chuck Zeiler collection. Number 6109 was built June 1914 (c/n 41489) and retired March 1952. The following is from the book, Steam Locomotives Of The Burlington Route, by Bernard G. Corbin and Richard F. Kerka: The successful operation of the first five 2-10-2 types prompted the Burlington to order additional engines of this wheel arrangement from the Baldwin Locomotive Works. Engines No. 6104 - 6125 were delivered in 1914 and were classed as M-2. Number 6110 was exhibited at the Panama Pacific International Exposition held in San Francisco during that year. The M-2 class incorporated a number of modifications compared with the 2-10-2's built in 1912. For example, a superheater of the Schmidt type was employed, a brick arch (supported on water tubes) was placed in the firebox, and an automatic Street stoker fired the boiler. A number of other detail changes were incorporated, but basically the M-1 and M-2 engines were closely similar in principal dimensions, weights, and hauling capacity. To reduce the dynamically unbalanced forces on the rails, two of the M-2 locomotives, No. 6108 and 6109, were experimentally fitted with light-weight reciprocating parts made from heat-treated alloy steel. As was done on the Class S-3 Pacifics, the piston rods and crank pins were bored hollow, and to further reduce weight, the Laird-type crossheads were adopted. By reducing the aggregate weight of the reciprocating parts by 379 pounds (or 16 percent) compared to the earlier 2-10-2 types, the additional counterweights that had been keyed to the main axles of the M-1 engines were no longer needed. Engines No. 6108 and 6109 were then classed as M-2-A.