Bumping this post about the Swiss transfer ticket.
How strict are they about only using it to/from the airport/border stations?
I'm doing ZRH-Zermatt-ZRH so it's no problem. But I have friends that I think are stopping in Zurich overnight either before or after Zermatt.
Is there anything (other than their consciences) preventing them from boarding/alighting the train at the Zurich Hbf instead of the airport in one (or both directions)? I don't recall fare gates, and I can't imagine there's a validation stamp (which wouldn't work for mobile tickets?)
Other ticket options seem to be much more expensive.