45' trailers didn't appear until the very late 70s or early 80s. The 89' flatcars spent the 60s and most of the 70s hauling 40' trailers.
Why they chose 89' over the earlier 85' I don't know.
40' trailers appeared in the mid 50s, so I doubt that many 32' were carried on 89' flatcars. They certainly weren't used for intermodal in 1971.