Train Consists

The load would remain on the same flatcar from origin to destination.

Curt

Curt  Thanks-that's what I would have "guessed"-but "knowing" is always the better way to go. Paul

Your mention of ship building parts. I had a K-Line flat years ago that had a huge boat propeller as the load. Was different and cool looking. I just never ran it much and eventually sold it.

The process you are inquiring about is called interchange.

There are designated locations at which a railroad interchanges cars.  Not every single track connection between two railroads is an interchange track.  The receiving railroad will inspect the cars before accepting them.  If defects are found, the car(s) may be refused, and it is then the responsibility of the delivering railroad to repair the car(s) and make a second attempt at delivery.

The comments above cover the basics of interchange of cars between RRs.

To add a little more information to what has already been said, the various different RRs had different interchange policies with one another.  It actually was fairly complex.  Some cars were operated in "pool" service.  Therefore it should not be assumed that the guns always shipped out on B&O flats.  RRs completed with each other, but also worked with one another to minimize shipping cost.  Don't assume that the car once unloaded in Maine, went back empty directly to Washington.   Of course the opposite applies, if say there was an empty B&M flat car in North Carolina, it might go to Washington to pick up a gun on the way back.  I remember as a kid watching C&NW and Milw freight trains (1960's) and saw box cars from virtually every RR at one time or another.