Just checked-google is as so often a good friend: goldenpassline.ch.

NO such dedicated trains INterlaken-Montreux for now, though in hi-summer that may be different. Covid has seen tourist nrs plummeting down.

The normal trains, with same views on lakes+mountains, run 365 days and ev 1 or 2 hrs-on that site you can also see current timings. You can use these for ´free´=without any extra to pay.

I suppose you are US and riding 1st? IF you insist on the special cars with high supplmt etc- just ask after arr. in the beautiful Swiss at any SBB counter (=national railways), though these lines are done by another regional railway (MOB and Centralbahn)-that does not matter, they all cooperate.

Normally you don't need a reservation in Switzerland. If you travel during high season you might want to make a reservation between Zweisimmen and Montreux. I travelled there during Easter and there were plenty of free spaces.

You can read more about the Golden Pass Line here:

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/#golden-pass-line