Hello niakm!The cheapest rail pass is the Swiss Half Fare Card, which currently costs CHF 99 per adult. It allows you to purchase your tickets for half the regular price. Purchasing this card would only be economical if you do more traveling than just a return trip Geneva - Interlaken. A standard 2nd class return ticket Geneva - Interlaken Ost costs CHF 130 per person. Children under six years old may travel along for free in any case; you don't need a rail pass for that. So I recommend to purchase two standard return tickets.I hope this helps. Enjoy your trip!
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