It wasn't sudden, and was already in progress by 1904, but St. Louis in the 1850s was the second largest port in the United States after New York. As railroads

It wasn't sudden, and was already in progress by 1904, but St. Louis in the 1850s was the second largest port in the United States after New York. As railroads became a thing they favored Chicago, possibly because St. Louis politicians were protecting the river boat industry. At the dawn of railroads St. Louis was three times the size of Chicago, by 1904 it was reversed. ␄ Caveat: I didn't vet those numbers too closely and in the case of St. Louis it matters. St. Louis is one of several US cities whose urban centers are politically separate entities from their surrounding suburbs. So when you see a list of "Cities with the highest X rate!" and X is correlated with population density then St. Louis and Baltimore will usually be high on the list.)