In the days before 9/11 one could, and I did, check my bag at SFO and pick it up off the carousel at Cologne Hbf. Today if you buy a ticket on Lufthansa from SFO to Cologne you have to pick up your bag next to the train station at Frankfurt Airport and carry it on the train yourself, but your Lufthansa ticket is good on the train. There is a Lufthansa agent on the train to check your ticket. Hub and spoke doesn't have to be only with airplanes.

Flying to Köln-Bonn airport would take much longer than the 57 minutes on the high speed train to Cologne. And then you would still be at an airport instead of across the street from the Cologne cathedral in the middle of town. The small hotel I usually stay at is five blocks away on a quiet street.