"The last BMT car (#153) first appeared in the ORER dated April 1959
while the first XT car (#154) first appeared in the July 1961 ORER.
Did something happen during that interval requiring a change in the
AAR class designation."
It would appear that the company was simply choosing to transition from passenger to freight service. There was no change in either of the listed AAR Classes or designations during that 1959-61 period.