Railbox came out when there was a major shortage of boxcars in the U.S. IIRC, it was a joint company of most of the railroads. Their feature was that they had no "home road" to return to. About the same time, the interchange rulers introduced "Incentive per diem" charges (extra daily charges) to induce roads/shippers to get cars moving again in a hurry. This caused a number of otherwise insignificant roads to suddenly acquire large fleets of boxcars.
The make up of a train would depend on whether it was coming or going, and which boxcars were on hand.
Some roads ran special trains to collect foreign cars and get them "off the property" and onto some other road so that the charges would stop accumulating.