There was more interchange of passenger cars in the northeast, because several trains ran their route over several railroads. Examples I can quickly think of were trains that ran between Boston or New York City and Montreal or Toronto, as well as through cars which left the New York area on Lackawanna or Lehigh Valley, and continued west of Buffalo on other railroads to Chicago. One who posts here rode a through Pullman that originated on the Lackawanna and arrived in Chicago on the Nickel Plate when he was a college student.
Storage mail cars were often interchanged at Chicago, and the secondary passenger trains operating east and west of there frequently had at least one foreign line head end car, and often had several.
Coaches and chair cars were not often handed off in interchange, unless the entire train operated over more than one railroad.
