It's mostly a function of where there are terminals that present opportunities for cross-dispatching. CN coming out of Chicago on CPKC is relatively infrequent because their traffic does not cross as many paths in those cities. But CPKC locos arriving in Bensenville from Canada and CN locos arriving at Homewood may wander just about anywhere in the US because of the number of run-through trains that originate there for foreign parts. CN and CPKC sightings are quite regular all the way to California and Mexico.
One frequently sees CN locomotives on the CPKC through the Rockies, because there are enough places in BC where power ends up at some other railroad's terminal.
And - once a locomotive gets on a unit train - it's a crapshoot where it goes next, as the power will just follow along with wherever that trainset is sent.
Sometimes the result is downright absurd. Here is a BNSF work train on the Sandhills Sub in Nebraska many years ago (those particular engines are long past retired). Who knows how that came about, or how long BNSF held onto that power. Definitely a double take when we stumbled upon it.
- Paul
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