I was using a Saver Day Pass (49SFR) and for my "
day trip" did ZRH-LOCARNO (new Gotthard Tunnel)-Domodossola-Brig (Simplon Tunnel)-Zermat-Visp-Zurich ("new" Visp Tunnel)
...So I did 3 of their MAJOR tunnels in one day.
The first time I used the Cent. line (shortly before EU began), going from Locarno to Dom. Italian PP agents did a PP check at the Swiss -Italy border; now at the border a vendor boards with a food cart for selling snacks/drinks.
PS: I just tried to do a "dummy" booking on
www.sbb.ch/en for Locarno FART to Brig--there is NO warning re boarder crossings.
You wouldn't expect a warning re border crossings as it wouldn't make any difference to what a passenger does (other than declaring certain items). As was said above, anyone in Switzerland and Italy should have ID on them at all times anyway, whether they're crossing the border or not.