Somebody needs to find pictures of these cars in service across mutliple eras... also need to narrow down variations as well.
What got me interested in a scene of an NKP Berkshire with one of its trademark hotshot freights on one of Emery Gulash's "Best of Steam" Videos, the first car in the train is a 50 ft. flat with a Lackawanna trailer, and right behind it is an ACL watermelon car.
If you look at the SAL-ACL-SCL Historical society's website you'll find lots of photos of these in various eras. ACL, SAL, and CofG were the only roads with large fleets by 1960, but they were often seen in Midwestern freights. The photo is an ACL B-17, but the SAL's were very similar. There are quite a few on George Elwood's site, too. Look for car type "VM".