Reservations on trains in Switzerland are always in addition to the cost of your ticket or pass.
You can make reservations and choose your seats here:
There will be a drop down box during the booking process, where you can indicate that you have a Swiss Travel Pass. As you are travelling in August, a reservation on the Glacier Express will cost CHF 43.00 for a long journey, or CHF 33 for a short journey.
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(Change the language to English at the top right of the page if need be)
Reservations for the sector between Zweisimmen and Montreux cost CHF 8 (or CHF 15 for the VIP seats in first class).
There will be a drop down box during the booking process, where you can indicate that you have a Swiss Travel Pass.
If you are also booking the Bernina Express bus, for best results do separate searches/reservations for the bus and train.
Reservations cost CHF 14 per seat per sector.
Gotthard Panorama Express
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You can't book the boat, just the train between Lugano and Flüelen (or vice versa).
To reserve the correct train, choose the train from Flüelen to Lugano, departing Flüelen at 14:10, direct to Lugano, and denoted by the letters EXT.
Or in the other direction, book the train from Lugano to Flüelen, departing Lugano at 9:23, likewise direct to Flüelen, and denoted by the letters EXT.
If you can't type the ü in Flüelen, spell it Flueelen instead.
Reservations cost CHF 24 for the whole trip. The Gotthard Panorama Express does not run on Mondays.