PRR had several thousands gons for steel use, stock yard service and general service. Chessie had mill gons and would fill in with Railgon if needed. Of course Chessie unlike PRR would load a foreign road car if there was a need.
A good P company man would never load a foreign road car before the home cars. Of course if Westinghouse needed 30 boxcars and I had 30 PRR empties and Strongs Manufacturing needed one guess who got the boxcars? Then if the load was bound for Chicago I might try to locate a empty GN,C&NW or R.I and send it to Strongs or I could send over PRR TOFC trailers for loading if Strongs agreed.
You couldn’t swing a baseball bat in Russell yard without hitting a IPD boxcar. IPD boxcars filled the boxcar shortage until the railroads rebuilt or bought brand new 50’ boxcars. Then when the IPD era ended railroads bought newly new boxcars at bargain basement prices.