I will be going to Switzerland in June and recently purchased a Swiss Travel Pass primarily for rail travel. I understand it is necessary to reserve seats on the on the scenic tourist trains such as the Gotthard Panoramic Express, Bernina Express etc. Can anyone direct me to a website where I can make seat reservations using my Swiss Travel Pass without buying a separate ticket. Every Google search site I've tried seems to be a dysfunctional deadend.
Glacier Express: https://www.glacierexpress.ch/en/
Bernina Express: https://www.rhb.ch/en/panoramic-trains/bernina-express
Gotthard Panorama Express: https://www.sbb.ch/en/leisure-holidays/trains-trips/rail-travel-specialtrains/panoramareisen/gotthard-panorama-express.html
The Gotthard Panorama Express is 1st class only for the rail portion. If you bought a 2nd class pass, you will need to purchase an upgrade ticket.