Seat reservations in Switzerland are generally not necessary. In long distance trains, seats can be reserved in some coaches but people rarely do it. That includes the high season (although in Switzerland, I'm not sure that's summer).

On the Rhätische Bahn, seat reservations are only possible in the panoramic trains, which, on the line to Tirano, also have normal reservation-free coaches.

Great Thanks so much. That"s good to know.

Do you start from Chur? may i found a nice connection for you with the roofless “Cabrio” on parts of your Route.

As said above by rvdbrgt you need a reservation for the “panoramic cars” the hauling unit of the Bernina Express can be used without any reservation and even have few openable windows (In direction of Chur - Tirano try to get seats on the right hand side in direct of travel for the bestview´s on Albula and Bernina Route.

If you want to reserve seats in the Panorama Cars then go to the Rhaetian Railway site (they are private & separate to SBB)….here you go - https://www.rhb.ch/en/panoramic-trains/bernina-express

Here you have all the information you need for a trip with the Bernina Express.

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/#bernina-express