Hi apheron
Thank you for oyur quesiton.
You can reserve your seats on Swiss trains at SBB.ch, SBB Mobile, via SBB Contact Center 0848 44 66 88 (CHF 0.08/min.) or direct at the ticket counter. It costs CHF 5 per seat to make a reservation.
Not all trains require a seat reservation, and on some, you cannot do it.
When choosing a connection, check if there is a “R” symbol: it means the seat reservation is possible. If there is not, it is not possible:
EleonoraS_0-1652186952117.png
As you can see here: for the Luzern-Meiringen section it is possible to reserve seats, for the Meiringen-Interlaken section it is not.
I wish you a pleasant day.
EleonoraS