Many boxcar types from the 30s and earlier lasted into the 50s. Here are some photos of trains from that era with many different sizes of boxcars in tow:
I'm trying to recreate that type of freight on my layout. I have several pre WWII style cars, including the MTH car Patrick has, but most are from Atlas O. Here are some of my pre WWII Atlas O cars.
X-29X-29 REA