I would argue the question this way. If I were looking at it from a U.S. carrier's chair—if I were the strategic planning individual at the Norfolk Southern—I wouldn't be interested in a Canadian carrier; I'd be looking west. You could have transcontinental mergers in the U.S. Then I could just pick the traffic I want off the border points because of extended interswitching, and I know I have no threat there.
A consolidation scenario, then, might leave the Canadian carriers out in the cold.