Some years ago, Lufthansa stopped flying FRA-CGN and also reduced the number of flights STR-FRA. Instead, Lufthansa is code-sharing on "flights" between Cologne and Frankfurt as well as between Stuttgart and Frankfurt with the DeutscheBahn, thus some trains have a LH flight number, you get a boarding pass instead of a train ticket and so on. This way, you don't need to buy a rail&fly ticket to get from Cologne to FRA, but you can simply book it as a flight, check-in in Cologne etc.