Yes, this is possible. Your Swiss Travel Pass equals a Day Pass or the GA travelcard.
As a consequence, you will have to pay the half of the difference between 1st and 2nd class. There are multiple possibilities to buy it, depening on the route, the one or the other might be more beneficial:
- a point-to-point class upgrade (at half fare - 1/2)
- a saver class upgrade
- a day class upgrade (half fare)
You can either buy it at any staffed train station, on sbb.ch/en or with the SBB Mobile app.
See this link to learn more: Class upgrades and Supersaver Class Upgrades | SBB
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