There's already a vast network of railroads in the north. The beef from the Chicago slaughterhouses has to get to the rest of the country somehow.
The Mississippi has always been a major means of transporting lots of goods and a source of our wealth. We don't need it though, if we can continue to borrow all the money we need.
Australia is a continent, but it's also an island. If you introduce any invasive species on an island, it wreaks havoc.