burock Okay, I understand, it’s a bit late though. Actually, what’s on my mind is whether we can find seats for the return trip.
Tickets do not give you seats. A ticket is just a receipt that proves that you paid for transportation. A pass is a ticket with wide geographic validity.
With a ticket you are permitted to be aboard a train. In most cases there are plenty of seats, but it does happen that more people board a train then there are free seats. Then not everyone will sit. Trains are just mass transit. What happens on a subway of more people board then there are free seats?
To ensure a seat you need a reservation. A reservation is something entirely different. Lives in a different universe. You do not even need a ticket when purchasing a reservation. (But you will of course be required to show one when on board).
You can get reservations on the regional trains on the Bernina Railway. Because the line has become a victim of overtourism it is a good idea to get those. You can get them here:
https://www.rhb.ch/en/tickets/bernina-line-reservations/