The U.S. doing business with Mexico over China is great. Some pros I can already think of:
With increased transport using rails, it’d be easier to smuggle shit, therefore you’d think the cartels would be more cooperative. Which means less violence, less turf wars, etc.
Better for the environment. Much less fuel to transport stuff.
If Mexico’s economy improves, a lot of the illegal immigrants that would normally come to the U.S. would probably go to Mexico instead (closer, similar in culture and language).
Pretty much can’t think of a single con, other than prices going up a little bit. It’s worth it.