The vast, vast majority were used in captive service for the transportation of auto parts. On my 1980 layout, for example, the Ford Fast travels from the east towards their plant in Milipitas CA, exchanging among a variety of railroads, but for my purposes, it flies across the region as MOPAC train from Kansas City, to be exchanged in Pueblo with DRGW to continue west. I'm guessing the other manufacturers had similar trains.
I have six auto parts boxcars already destined for this Ford Fast, *four* of which are the 86' units such as you have: Louisville & Nashville, Missouri Pacific, Detroit Toledo & Ironton, and Chicago & Northwestern (the latter two of which were undecorated and I used oddballs decals).
Here are three of 'em in a pic I did for my parents-in-law (the quad hoppers were bought with Xmas money from them, notice the XP's equal the length of two of 'em almost exactly) . . . . .