Operating Era: 1911-1955. Chicago, Burlington & Quincy, Ahnapee & Western, Fort Worth & Denver, Kewaunee, Green Bay & Western. Burlington installed 11,500 cars, the largest classes ever on the railroad, in 1910-14. Although the cars had eight truss rod, they also had a steel center sill consisting of two channels running the length of the car. This enabled them to run decades later than wood framed truss rod cars that were outlawed in interchange in the 1930's. As built the cars had double sheathed sides and ends, arch bar trucks and wood roofs. The XM-17 and -18 were identical except for a small difference in interior length, reflecting the use of larger timbers in the later class. In the early 1920's the cars were given plain mullion roofs. Beginning in 1925 the cars were strengthened with reverse Murphy ends and wood reinforcements on the doors. About 1929 reverse dreadnaughts were shown to be an improvement over corrugated ends and these were used in subsequent upgrades. However, the Depression reduced the need for box cars significantly, and those cars not yet upgraded were scrapped. In the mid-1930's the arch bar trucks were replaced by Andrews. In 1937 the early slogan was added to the doors of cars when shopped, requiring the removal of the door braces. Car shopped in 1942 and later received the later slogan and AB brakes. As late as 1940 the cars were the largest classes on the railroad. After World War II they were rapidly retired but some lasted as long as 1953.