Andy,
Since February of 1949 through March, 1952 only two cars from the 28000 - 28499 series remained in service when one was retired.
The single remaining car lasted all the way into January 1955.
At the same time the 62000 - 63499 boxcars had also declined to a single example by November of the same year. (1955)
66000 - 67499 boxcars numbered 286 by the end of December.
Of course some boxcars were now in M of W service converted to bunk and supply cars and do not show up on the revenue car lists.
John T.