Per AAR interchange rules (my copy is 1975) per rule 90 (cars banned for interchange) under sub-section 3a, " all freight cars 46 years or older from date built or rebuilt... (at the time 1929) and sub-section 3b "Cars built prior to April 1,1950 equipped with Duryea underframe"
I also note that the FRA bans freight cars (including cabooses) over 50 years of age...
