Bruce Smith wrote:
"There may well be a census of X29s with AB brakes at some time in their history other than construction but it was almost certainly NOT done on a yearly basis. Why would it have been done in 1934? There was no pressure at the time to convert existing cars to AB brakes."
There certainly was a census of X29s converted to "AB" brakes along with ALL cars so converted. Upon the adoption of The "AB" brake system as "standard" by the American Railway Association on August 16, 1933, Rule 3, Section (a), paragraph 4 was modified to require all new cars built on or after September 1, 1933, to be equipped with air brakes meeting the air brake specifications as revised in 1933.
On November 20, 1934, along with the adoption of the original date for full conversion of all freight cars used in interchange (January 1, 1945), the Association of American Railroads membership approved the Mechanical Advisory Committee's recommendation to the Federal Coordinator of Transportation (Joseph Eastman) that: "A quarterly report to be made to the coordinator and the Interstate Commerce Commission showing the number of freight cars acquired and the number of freight cars on which brake equipment is converted each month."
The first quarterly reports were filed on April 1, 1935 which showed a total of 26,182 (1.26 %) railroad owned and 1,389 (0.52%) private line cars equipped with "AB" brakes.
In 1939 the ICC relieved the railroads and private car owners of some paperwork making the reports due semi-annually due on June 30th and December 31st (yearly).
Thus, all railroads and private owners submitted these reports thru June of 1953.