The reason why there were Railbox boxcars with other railroads reporting marks was Trailer Train Corporation had decided to return several of the surplus cars in the fleet to the member railroads (which helped finance the cars in the first place). This took place beginning in 1982, which by that time, the boxcar shortage that begin in the early to mid 1970s had turn into a huge glut for Trailer Train (and others like National Railway Utilization and ITEL Rail) .
IIRC Southern, Seaboard System, Richmond Fredricksburg & Potomac, Chesapeake & Ohio, Southern Pacific, Santa Fe, Burlington Northern, and Missouri Pacific were some of the railroads that received them.
Robyn (yay, my first post!!!)