The sbb.ch train site does not seem to take reservations, sell tickets, more than a month ahead. Does that sound right? I can go through raileurope site but much more expensive there.
Where are you traveling to/from and on what date?
Unless you are booking a cross-border trip to Italy, France or Germany and getting a nice discount, you do not need to book tickets in advance. Reservations for travelnwithin Swotzerland are not required, and apart from the primo seats in the panorama cars on one of the named scenic trains, you won't have any trouble finding seats. If you are concerned, you can book a few days ahed of travel when you arrive in Switzerland. Also some of the tickets sold by SBb have to be shipped to you and that is expensive.
Traveling from Rome to Milan to Tirano to St Moritz all in one day (May 6), then St Moritz to Zurich on May 7. We'll get in to Tirano at 2:50. Want to be sure we will get 2 seats at 3:40 pm on the Tirano to St Moritz since it's the last one of the day. It's not the Bernina Express, but same route. So you think no problem getting seats on short notice?