Cmm22 as I am afraid of arriving at the train and there being no space.
If that were to happen you will have a story to tell your children and grandchildren for the rest of your life. “I was in Switzerland, and could not get on a train because there was no space”.
You are given the option of the Saver Day Pass because for the trip you want to take it is the currently available cheapest option. And I would take it, as it is a very good offer. It gives you full flexibility, which is especially useful if you are arriving by train, and you can take all public transit. So you can change your plans, choose to take a slower, more scenic route etc…
Very few people bother with reservations on domestic trains. Trains here are mass transit. You go to a station, find your train and get on. Then on the train you show your ticket or your pass when the conductor passes. That does not always happens btw, so do not be surprised if nobody wants to see your ticket for a whole day.
And occasionally it does happen that there are more people on board a train then there are seats, and thus some people have toe stand. But that is rare, and usually you only stand till the next station, as there are always people getting of.