Re: GA Pass, Panoramic Trains and Mountains!
The GA is valid on all public transport in Switzerland. That includes quite a few mountain railways and even some cable cars.
As a general rule of thumb: Anything that serves a place that is inhabited year round is considered public transport.
So yes, it is valid on the "Montreux - Oberland - Bernois" as this is just the local transportation company. "Golden Pass" is just marketing fluff. It is also valid till Wengen on the Wengeralpbahn, till Mürren, both on the Cable car up from Lautebrunnen, as well as the Stechelberg - Gimmelwal - Murren part of the Schilthorn bahn. And you can also take lakes steamers, and go up the Rigi (from both sides). It is one of the best deals in Switzerland in my opinion.
On mountain railways that are not part of the public transport network (eg the Wengeralpbahn beyond Wengen, or the Schilthornbahn above Mürren) the GA does give you 50% off. Still a good deal .