Hello Omid
Please kindly note, this platform is not a customer service. We are a customer help forum run by volunteers. We can guide you and give you links to reserve seats, but we cannot make the seat reservations on your behalf.
First of all, I would like to point out that you are using the Swiss Travel Pass, an all-in-all ticket for tourists. The SwissPass is a product for Swiss residents; you should not mix them up, they are not the same.
Neither the Bernina Express nor the Glacier Express is operated by SBB. They are run by privately owned railway company, Rhaetian Railways (RhB). The Swiss Travel Pass is valid on their trains, however.
Here is the link to reserve seats on the Bernina Express. The seat map allows you to choose the seats:
https://shop.rhb.ch/en/bernina-express
If you need help, please contact the Bernina Express customer service team:
+41 81 288 65 65
railservice@rhb.ch
Here is the link to reserve seats on the Galcier Express. The seat map allows you to choose the seats:
https://shop.glacierexpress.ch/gex_en/specials/daytrips/glacier-express
If you need help, please contact the Galcier Express customer service team:
+41 81 288 65 65
railservice@rhb.ch
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What concerns general advice on seat reservations on Swiss trains: it is not mandatory and often not possible. Our trains are free-seating. You can read more on that in our blog:
https://community.sbb.ch/blog/23887-demystifying-seat-reservations-on-swiss-trains-a-comprehensive-guide
Please let us know if you have got further queries.
Best wishes,
Roland