Having just read Trains' puff piece about KCS...

Assume mergers go down to the final round. As noted by WRJ:

UP gets CSX or NS. CSX might be a better fit from a management standpoint, but NS is the eastern US Borg - might make sense for UP to absorb them for that reason alone. Yellow & Black locomotives.

BNSF gets whichever one UP doesn't take.

CN already has coast-coast in Canada and the old IC route down to New Orleans, along with considerable coverage in the Midwest and the Rust Belt. They could probably get access to Mexico, and a port on each of the US West and East Coasts, over trackage rights from one of the Big Two mergers.

CP is odd man out, and so is KCS. While CN could remain viable with some added trackage rights, CP just isn't big enough. So I'd predict a CP+KCS combination making for the true NAFTA railroad (all 3 countries, 2 major West Coast ports (Lazaro Cardenas and Vancouver BC), access to the Atlantic at 2 large ports (Veracruz, Montreal), and access to the Chemical Coast). Both railroads have a lean culture and good operating ratio; main question would be: which management takes over?