Hello Sietueneec56
You could only reserve a seat on the IC train, not on regional services such as RE, R, or S.
A Swiss would not reserve a seat for this trip, not for two people.
If you still want reserving a seat:
SBB Mobile App:
- go to tab ‘Plan’,
- enter from ‘Zuerich HB’ to ‘Interlaken Ost’,
- select the required connection,
- select tickets,
- enter passenger data using the pen displayed above the itinerary & save,
- scroll down to option ’Reservation only (CHF 5.00 per passenger),
- choose the seat,
Kind regards,
Roland

