When I was choosing flights coming back to Valencia from Stuttgart, I noticed something interesting: there was a flight around 6:30pm to Zurich, connecting to a flight to Valencia; there was ALSO a train around 6:30pm to Frankfurt, connecting to a flight to Valencia. While the trip time was about the same for both, I would have had to get on an airport train from Hauptbanhof at around 4:30, giving a 2 hour advantage to the train itinerary.
Traveling with "Lufthansa Express Rail" is as similar as possible to boarding a flight as possible. First, there are Lufthansa self check-in kiosks on the ground floor of the train station, in the Deutsche Bahn travel assistance center, the "Reiseservice"...