OK, some more info for you.
It depends on the type of wood box car. Many box cars with steel frames, ends and roofs were built with wood sides during WWII to conserve steel. These cars lasted well into the 60’s - consider 20 years about the life span of a railroad car between major rebuilds.
Cars with steel underframes but other wise all wood construction would be rare after WWII, but did exist.
Ice reefers did MOST of the reefer work until the early 70’s! Yes, it took a while for mechanical reefers to catch on. While the earlest ones were built in the very late 40’s and early 50’s, wholesale construction of them, and replacement of existing ice reefers, did not begin until the late 50’s and early 60’s - again keep in mind the 20 year life span - railroads don’t and didn’t trade perfectly good rolling stock in like trendy people buy new cars - they used them until maintence or regulations forced them to replace or rebuild.
And, same as box cars, reefers with all steel construction except for wood sides would still be seen all through the 50’s.
I model the early 50’s, and actually, nobody currently makes models of any of the few mechanical reefers that were around in my 1954 time frame. Those 50’ and 57’ ones commonly available did not show up until '57 or '58, and even then it was only a few.
I know you did not ask about this, but I will give you a time frame for some other stuff:
Piggyback, using 40’ and 50’ flats started right after WWII and by the early 50’s was going pretty strong. 75’ piggyback flats were indroduced on a few roads in '53-'54, by ‘58 the 85’ flat was being developed and tested.
Tank cars moved lots of petrolium products until after WWII, during the 50’s this declined as ships carried more (through the Panama canal) and pipe lines were built around the country.
Automobiles were carried in 50’ double door box cars (6 autos to a box car with special racks inside) well into the early ‘60’s. 50’ open 6 c