Hello Guenecka72
You need seat reservations only for the panoramic cars of the Bernina Express trains, it is not necessary or possible for other regional trains. You can reserve seats here:
https://shop.rhb.ch/en/bernina-express
Please note that discount “Reservation only” can only be specified when the travelers are entered. Up to this booking step, the full price without discount is displayed.
If you bought a Supersaver Ticket, you can take only the train mentioned on the ticket.
Kind Regards
Danhiel