Well, Harry, while I can't point to any particular cars in N scale, for the early diesel/late steam era you would have seen reefers in 36'/34', 40', and 50' lengths, (the latter ones would most likely be express reefers, but not necessarily exclusively). Construction would be wood for the 36'-ers, and wood or steel for the longer cars. Ice cooled reefers lasted on the PFE until 1971, and many of those cars could have been of wood construction.
Depending on the year, cars might still have truss rods - all-wood frames (with truss rods) were banned from interchange service in 1940, but truss rod cars with composite (wood and steel) frames were allowed in interchange until January 1, 1952.
On most models, it would be difficult to determine the nature of the underframe's construction, so if you like truss rod cars and are modelling pre-1952, by all means use 'em. (And, of course, if you like those types of cars and are modelling a later date, no reason why you can't bend the "rules" to suit your own enjoyment.)
Wayne