The portion of the route you are travelling on has two track gauges so you travel Montreux to Zweisimmen on one actual train, then change from narrow gauge to standard for the balance of the journey to Interlaken Ost. If you were completing the Golden Pass route to the lovely city of Luzern you would change again to narrow gauge.
The Golden Pass, like the other scenic routes in Switzerland, are served by both sealed window, air conditioned, reserved trains for tourists and tour groups; and more frequent normal trains with windows that open very wide, slightly harder seats, no a/c, and plenty of locals - no reservations.
Your choice...
I remember travelling to Montreux one morning at go-to-school-time a few years ago on the normal train and being amazed hearing all the children chatting in german, then they got off and the next bunch all chatted in french. It was a wonderful day!
You may be able to guess which trains I prefer?