You missed his point. It's not about outsourcing "border security." It's about outsourcing someone who sees if the cars are properly attached to each other.
No, I believe they inspect the contents of the car and the seals on the cars at the Border.
Do you think the Mexicans will check for the presence of illegals or drugs, or worse ? Do you think those Mexicans won't take bribes for smuggling, or succumb to the threat of violence from those in Mexico who have an illegal cargo to get across ? Shoot, we can't even keep our union guys from pilfering, so why do you think we can keep Mexican workers attentive to American national security ?
I don't think it's possible.