Phred's caution applies to multi-country passes, but not to the Swiss Travel Pass. Since it's only good for one country, there's no issue of which country's rail system you used. You just show it when asked for your transit tickets, or upon entering covered museums, or when buying tickets for rides not fully covered. I was told at the time of purchase to take a picture of my passport info page, so I could have this to show and not have to pull out the passport itself (technically it's tied to your passport number so they need to see that as well as the pass itself). However, not once was I asked to show my passport with the Swiss Travel Pass (no guarantees for anyone else, of course).