Golden Pass Question

If buying tickets, you buy them for whichever specific leg of the trip that you want. There's no roundtrip discount and I find it easier to book each way separately if there's no reason to do otherwise. http://www.goldenpass.ch/en/goldenpass/horaire/index

It covers the return trip if you pay for it. Twice the cost of going one way.

The Golden Pass train is an extra tourist train. That rail line, like all others in Switzerland, has at least one train per hour in each direction from early until late.
Tickets are sold from A to B, you can buy a ticket from Interlaken to Montreux and it is valid for that trip on any train on that route (not just on the "Golden Pass" tourist train). Or you can buy one for any of the intermediate stations.
If you want to come back, you need another ticket from Montreux to Interlaken.

Thank You!!