PTC is relatively easy: non-equipped units can't lead, but should still be usable in consist.

Emissions: I doubt that the locomotives delivered in Mexico/Canada are without emission controls. They might not be T4, but T3 is likely. Might be worth looking at past CP/CN/KCS practice (if somebody around here has direct knowledge); when T3 was the standard here, what did they buy there?

If simply buying a locomotive by a non-US subsidiary allowed a railroad to ignore all equipment regulations in US operations, UP and others with access to the borders would be setting up shop in Canada/Mexico yesterday to do that. They're not, yet, so I don't think, logically, that it does. Not sure how it works though.

And, actually, there's a difference between this scenario and the NAFTA truckers. KCS is actually a US corporation that has a Mexican subsidiary, so US rules apply to them here. Most of the NAFTA truckers are domiciled in Mexico (or possibly Canada) and simply operate here under treaty terms. They don't, assuming adequate inspections, get a pass on major equipment safety rules. Emissions may be another story, though ARB "smoke tests" them like anybody else and issues tickets for violators (not sure if they accept pesos for fines).