Golden Pass help!

The MOB site should show the "official" Golden Pass connections.
You don't just "change seats" at Zweisimmen, you change trains. The train from Interlaken Ost is a standard gauge train. The train tracks from Zweisimmen to Montreux are 1 meter rail-to-rail, narrower than standard gauge, so the train from Interlaken can't run on them, thus the need to change trains.

The "Golden Pass" trains you are interested in have panorama cars between Zweisimmen and Montreux, so those are the trains people want to have reservations on. There are no panorama cars on the trains from Interlaken to Zweisimmen. The trains are all co-ordinated so as one arrives at Zweisimmen from Interlaken, the other train is waiting for its passengers to make the change.

So if you get reservations for one of the panorama trains, just make sure you get a train to Zweisimmen that arrives before the train you have reservations for departs.

That is very helpful. So I can plan on any train from Interlaken to Zweisimmen in that case? It doesn’t have to be a “ Golden Pass”? When I look on the Golden Pass ticket site it only gives two options for trains from Interlaken to Zweisimmen: 9:08 or 13:08.
I couldn’t figure out why the trains were so infrequent. But if I look on SBB, there are several offerings.

So my plan would then be to get on any train that my Swiss Travel Pass covers to get from Wengen to Interlaken to Zweisimmen, but prepurchase the ticket from Zweisimmen to Montreux?

Thank you, Sam!

Your Swiss Travel Pass is valid for the train from Zweisimmen to Montreux. If you want to be assured of sitting together, you should buy seat reservations.

If you're searching schedules through SBB or DB, it's helpful to add Zweisimmen as a midpoint on your route search. Or make separate requests to and from Zweisimmen. Otherwise, they focus on faster connections via Bern, which you don't want.