Golden Pass train ride

No, it goes both ways. In fact, it is not even a single train, it is just a name put on a general route by the Swiss tourist people. Montreux to Zweisimmen is a narrow gauge railway line. Zweisimmen to Interlaken Ost is standard gauge rail line. Interlaken Ost to Luzern is narrow gauge rail line. So it is 3 different rail companies operating each section, and you have to change trains at each point. Regular Swiss trains run hourly in each direction.

Use the trip planner at www.sbb.ch/en and put in your starting point, ending point (Muerren), date, departure time and via "Zweisimmen" to see the schedule.

Thank you !

I assumed as much, and I will try inputing the routes as you noted.

I would like to re-emphasise something Sam said. All train routes in Switzerland operate at least one train per hour in each direction, often more. There is no such thing as a daily or one-way train.
And tickets are valid on any train. There is no need to reserve or book seats (apart from the 4 "extra" tourist trains), just get on the next train.

I believe there will be some track construction on the Golden Pass line this year which would require you taking a bus for a small portion of the trip and keep you from making reservations on the Golden Pass train. It should be well coordinated, with buses waiting at the station, but be aware there is a change in the usual pattern of travel.

"There is small construction on the Golden Pass route yes: between Montbovon & Château-d’Oex. It will require you to depart the train and board a bus and then rejoin a train after. It is a very small part of the trip (about 10km) so I wouldn't let that stop you from doing the trip.