I was checked by customs officers in Basel while coming from Brussels. First I was asked if i had beverages and cigarettes in my baggage i said no. Then they wanted to see my ID. I gave them my "Aufenthaltsbewilligung" issued by Swiss migration bureau, which did not suffice them (I did not shave for three days at this time), and was asked for my passport. My passport was damaged in the right down corner after they presumed the plastic cover had double layers. They looked into it for at least 20 minutes, calling everywhere and checking my identity (Yes, I have a non-EU citizen passport), and detached the plastic cover.

I asked them to write an "Akt" that they have damaged my passport. After they tried to discourage me from that by telling i would have to get off the train and that the passport was authentic, and even if the plastic layer is detached from the paper underneath, it is still ok to use. I said no, and I got off the train and have them write that protocol. At this time you can imagine how much humiliation it is when the whole train starts looking at you and thinking whatever they think. The custom's officers further tried to discourage me in their own office, starting running around, shitted their pants, which, I suppose, means that their behaviour was not quite legal, and after almost 30 minutes they gave the paper saying during the check that plastic "hat sich gelöst"...just like that, by its own...

So, yes one still needs a passport when entering Switzerland on the train.