The 40 steel box car so familiar to model
railroaders was a product of the 1930s. Wood box cars, which were built
into the World War I era, and early steel cars were largely
non-standardized, with details varying from railroad to railroad. The
move toward standardization began with American Railway Association
(ARA) designs of 1923 and 1932. It culminated in the 1937 AAR boxcar,
which was adopted by railroads from coast to coast and built in the
tens of thousands. (The AAR (American Association of Railroads) was and
still is the successor to the ARA.)