If Chicago didn't exist, someone else would have put a city within a handful of miles of the same location.

It's about as centrally located to the US as you can get from the Great Lakes, and once a canal opened (in this case, the Eerie, but shorter canals could have existed, it was just in NYC's interests to have that particular canal), then that's about as far as a boat can go, and water traffic is more efficient than rail.

So you would have trains leaving 'Chicago' to go everywhere, and someone would have thought to build a canal to get to the Illinois river and hence the Mississippi.

I'm not so sure that "St Louis" had to be exactly where it is, but Chicago is virtually a foregone geographical conclusion.