Several years ago I travelled IAD-FRA-Stuttgart train station on a LH air-rail ticket. I remember having to retrieve my checked bag in a special area near the entrance to the long-distance train station at the Frankfurt airport and being required to travel on the train in a car explicitly reserved for LH passengers. I assume that my ticket was validated by a conductor on the train, but I no longer clearly remember that happening. I flew back to Washington from MUC and did not have a rail-air ticket for the return flight.