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Yes, there is such a thing named => Saver Day Pass, check the availability for your travel date.
Note that this ticket type is basically not refundable in case of a change of plans.
Also note that seat reservations for the Bernina Express (if applicable) would need to be booked separately on the operator’s rhb.ch website. But there are also a few coaches on the Bernina Express train (= PE in the timetable) without seat reservations, and also other R or RE trains on the same Bernina route without any reserved seats.