For trains travelling **entirely within Switzerland**, passport/visa checks are not a routine part of the journey, though police checks can occur. I couldn’t find a reliable published frequency, so I wouldn’t put a percentage on the chance of being checked.

There’s a separate reason to carry ID: **if you use a Swiss Travel Pass, you must carry your original valid passport or official identity card** and present it during ticket inspection. A photocopy or phone photo is explicitly insufficient. [SBB’s rules](https://www.sbb.ch/en/offers/swiss-travel-pass).

If your train crosses into a neighbouring country, take your passport—or a national identity card accepted for your nationality—and any required visa/residence documentation. Switzerland’s document requirements depend on nationality. [Swiss migration authority](https://www.sem.admin.ch/sem/en/home/themen/einreise/faq.0064.html).

My practical advice: carry your passport securely unless you have an accepted original ID card that covers your journey and ticket.