Any road could (and can) be found anywhere, of course. That's what "interchange" is all
about. Real RR's are not "train sets".
If a load of XYZ was loaded in Albany NY, using a NYC boxcar, it wouldn't be off-loaded into a WP car in KCMo. The NYC car would show up in Los Angeles...
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I used to be a RR clerk, then programmer. My "railroading" wasn't the kind out in the yard and on the main.
Railroads run on paper, not rails; the trains are just a method of generating more paper,
of the greenback kind, the forms kind and the now-common virtual kind
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Demurrage - every RR charges every other RR for demurrage - the length of time RR A keeps a car owned by RR B on RR A's tracks. This, of course, sends cars back to their "home roads" from the "foreign line" (proper RR term - other RR's are "foreign") as quickly as is reasonable.
So, if the NYC had a "car request" from an industry in Albany, for a shipment to "settle" (delivering carrier) on the ATSF in LA, to the consignee there, and the NYC had an appropriate ATSF "mty" (empty) car in the yard at Albany, they would load the ATSF car rather than a NYC car to get the ATSF car off their line as a loaded car - making money from using the ATSF car for the shipment and also no longer paying the ATSF for their (ATSF) car sitting on NYC tracks.
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But everything goes everywhere - two of the most common roads one sees on cars in old RR films are those of the Louisville and Nashville (L&N) and the Southern. Any layout of the 40's should have some of these, regardless of location. Lots of NYC and PRR, too, anywhere.