W/a POD after the American Civil War, develop a plausable scenario where the U. S. national capital is moved west to Chicago, St. Louis or someplace more toward the center of the westward expanding nation.
After the ACW, Thande.
I think the OP meant a POD after the OTL resolution of the Civil War, guys.....
Short of a coup or a nuclear war, I'm not sure what else could cause them to move the capital of the US anywhere farther west.
Correct me if I'm wrong, but as far as I know, there is no real precedent for moving a capital outside of war or a coup.
Nigeria and Brazil, IIRC.
How about a particularly nasty epidemic (that stretch of the Potomac is a swamp) which kills off a large number of elected officials? The survivors decide to find another place for the seat of government, and figure why not look for someplace more centralized in the expanded territory?
Easy
1] if Post ACW- Confeds won, and The US moves the Capital away from the Border.
2] 1911 free trade agreement with Canada goes thru, US/ Canada merge in 1930's, Capital moved as part of Merger.
3] US Allies with the CP in WW1, takes Canada, The USC, needs anew Capital for the New Nation.