My experience is, if you have a reservation, you lose your 'reserved' seat, but you can still get on the train and 'search' for an empty seat.
If you read the rules, they may say something else. And in fact I was confronted with this very problem, and felt very intimated by a gestapo type German Train employee working in the 'information office', who was hell bent on enforcing rules, which would have amounted to a $200 change fee.
I quickly thought better of it, and having a reasonable amount of train traveling experience, and just boarded the next available train. Conductor came by, punched my ticket, was a total non-issue. This was from Frankfurt airport to Amsterdam, so not a terribly short trip either.
Worst case scenario, they would make you get off at the next stop, hopefully by then, the next stop would be St. Anton, and perhaps they would try to issue a fine. Not that they would, but that they could, but as long as you have a valid ticket for that day, I don't think the people punching the tickets on the train really care.
However, if you had no ticket at all, that is something entirely different.