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You do not have to ride an entire famous railway to enjoy it. The GoldenPass Express section from Spiez towards Gstaad brings the Simmental and Saanenland into one outing, with time for a village walk before returning. Choose an actual departure for your date.
Compare the complete outward and return journeys, including connections from your accommodation. A transport ticket or valid pass and a seat reservation are separate things. Check the chosen class and train’s rules before buying, and allow for timetable changes or engineering works.