And, on the Big Sandy line in WV, UP locos are very common on the NS. Maybe they have some type of pool agreement?
Actually, I've seen almost every major railroad on that NS line, sometimes more than one visiting road on a train. Even CSX shows up on rare occasions.
The best was a nine-unit consist with NS, UP, BNSF, CP, CN, and CSX. I can only remember seeing one KCS unit, and no Mexican roads.