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Take the GoldenPass Express from Spiez towards Gstaad for a day spent looking out of the window. The route runs through the Simmental and Saanenland. Get off in Gstaad for a walk, something to eat and the train back. You do not have to ride the whole line to Montreux to enjoy it.
Before booking, check the train times in both directions. A seat reservation and a ticket are separate purchases, unless your booking explicitly includes both. If you already have a travel pass, check its validity for this route and class.