About the Burlington between the Twin Cities and Chicago...generally the trains remained intact so cars weren't mixed together. So like a Great Northern train from Seattle would pull into St. Paul Union Depot, the GN F-units would be cut off, and CB&Q E-units would couple up and take the train the rest of the way to Chicago. Same for Northern Pacific trains.
However, especially as trains got shorter in the 1960s, there were situations where say an NP train and CB&Q train would be combined. I know the GN Western Star was split up in the Twin Cities - the mail cars it carried (after 1960) were moved over to the Milwaukee, who had the mail contract Twin Cities - Chicago, and the GN passenger cars were combined with a CB&Q train (I think the Blackhawk?). So you could see a Milwaukee RPO or Baggage car with mail on GN, or GN cars on the Milwaukee, and a block of GN cars in a Burlington train.
