This would have a huge impact on St. Louis as a city. It's very possible that the 1893 city-county divorce would happen early with the cession of land to form a federal capital, but that also likely means that St. Louis itself gets a much more densely populated urban center due to a desire to be in the proximity of the federal government. I would assume that some kind of l'Enfant-style rebuilding project would be carried out to turn St. Louis into a national capital, which would have an interesting impact on the cityscape itself - would they want to emulate the grand boulevards and Palladian architecture of the monuments of DC, or perhaps it becomes a Queen Anne/Italianate/Neo-Gothic mishmash that only the gilded age could pull off. Almost certainly it becomes a hub of railroads, and perhaps this would impact Reconstruction and southern industrialization with railroad magnates connecting a triangle of nearby factories, the federal capital, and Texas oil fields.