Apparently it wasn't just the SP. Looks like the Burlington Northern, Rock Island, and Milwaukee road tried them at least once.
http://bnsfsd70.rrpicturearchives.net/showPicture.aspx?id=1891538
SP developed the car and concept (i.e, NS TopGon, UP Autoflex, etc)...wasn't meant to be taken as the only road to have them... sorry.