Most likely opportunity was roughly 1775, during the War of Independence. One thing that immediately jumps out to me is that there would be no Erie Canal, as control of Canada makes the St. Lawerence Seaway the more viable vessel. As such, NYC remains a large city, but not the defining one for the American East Coast. Boston, Montreal and Quebec City are thus far larger. I wonder if, ITTL's future, things like NAWAPA could be attempted.
