The possible Bering Straits rail tunnel has been discussed…most people see it as economically unsound. But what a bout a direct rail link from El Paso to South America?
It would be technically difficult (the 40 mile Darien Gap would be expensive to cross).
But once in place, a rail like could take the place of thousands of trucks. Would such a rail link make sense? The USA-Mexico trade is enormous-not sure about further south.
I’m pretty sure they already have a steady stream of trains going to and fro from SA and the States.
So what is the current legal trade between North and South America? And where do you get the idea that a rail line would replace thousands of trucks? The Darien Gap precludes that already.
Goods go south, people come north. What could go wrong? 
There aren’t even any real roads between North and South America and it isn’t because it is impossible. The Darien Gap between Panama and Colombia has been left that way intentionally for a few reasons but one of the big ones is the political decision not to encourage free and easy transit between the two continents.