On some layouts I have seen a variety of road names being pulled by a single engine. Think a NYC engine pulling NYC, PRR, B&O, etc. cars. How common was this prototypically? How would you know what cars would reasonably appear in a train while others wouldn't for a given railroad? Is it all guesswork, or did agreements exist between the different railroads to lease/"borrow"/etc. each other's cars?
I'm not quite sure where this topic fits best but since I'm thinking of modeling prototypical train consists I'll stick this under the 3RS forum.