>>Given a selection of boxcars suitable for grain service would a railroad choose to provided their customers with their cars or cars from a foriegn road?<<
Well . . . . . . That depends.
If it is during the seasonal grain rush, neither the railroad nor the shippers care about whose car gets loaded with grain. In the upper midewest/Great Plains, large volumes of grain were shipped from the local grain elevators to the mills in the Twin Cities or to storage facilities in Duluth/Superior for eventual movement down lakes. The volume of grain shipped during the grain rush was so great that Car Service Directives were published directing all railroads direct all cars capable of carrying grain regardless of roadname to the grain harvest areas for the duration of the grain rush.
It would have been rare (but not impossible) for an off line mill to purchase the equivalent of a single car of grain from a specific elevator.
This is a case where it pays to study and understand the traffic patterns for specific commodities.