I'm going by train to Frankfurt airport. Everything is regional trains except for the last leg from Frankfurt Main Station to the airport. This last leg can be done on a local train (S-Bahn, RE) or on a high speed train (ICE). Ironically choosing a high speed train enables "Sparpreis" (saver ticket) which makes the whole ticket substantially cheaper. We are talking half the price here !! (Bizarre pricing, but that's what it is).
"Sparpreis" requires you to take the train as booked ("Zugbindung") for the ICE. Taking a different regional or local trains the same day is fine.
I was considering not taking the ICE since local trains are more convenient and I can hang around Frankfurt downtown a bit (I got plenty of time, if there are no delays).
Question: Can I take the local train? Does not taking the ICE somehow invalidate my fare since the 50% discount is specifically tied to the ICE? If yes, how would Bahn actually know and what would they do?
UPDATE Given the comment I'm trying clarify the question.
Let me describe it the following way: The price to Frankfurt Central is $50. By adding another leg from Central to the airport the total price drops to $25 but ONLY if this extra leg is on an ICE/IC train. I know, this makes absolutely no sense, but that's what is.
The situation is similar to "hidden city ticketing" (or skiplagging) with flights. Often flights from A->B->C are cheaper than just a flight from A->B; even if it's the exact same A->B flight. So customers for A->B just buy A->B->C and simply get off at B. The airlines take a very dim view of this practice and have (on rare occasions) even sued customers over it.
Just as an example: Lufthansa is one of the suing airlines and for air rail tickets they try to enforce this on the Bahn as well. You are not allowed to skip the train portion of a trip. People still do though, since LH has no legally defensible way of proofing whether someone was NOT on a train (unless the passenger does something stupid).
My question: Does Deutsche Bahn has a similar concept to "hidden city ticketing" (as Lufthansa does) and, if yes, do they have any rules and/or enforcement around it?