Golden Pass Panoramic train out of service until???

There are trains on that and every other Swiss rail line every hour 365 days per year. The extra Panoramic train is being refurbished and will be back in service in time for the summer season. I don't know exactly when.

There are numerous other passenger trains that run the Golden Pass route. They are just as scenic and they do not require a reservation. You can just get on the train (with ticket or Rail Pass) and go. I was on it in October and it was beautiful. Incidentally, with some of the other panoramic trains, such as the Glacier Express, unless you are part of a group, you may have difficulty getting a reservation on them (even if you have a rail pass). To avoid disappointment, make sure you check (station ticket offices can usually help you, if they can) and be prepared to use alternate trains and/or routes. For example, I took the Domodossola–Locarno railway from Locarno to Domodossola instead of the Glacier Express, and it was quite scenic. Also, instead of the William Tell Express, I rode a regional train along the route. Again, it was just as scenic (I passed through at least four spiral tunnels) and I didn't need a reservation. I used my rail pass.

That Locarno-Domodossola route is the Centovalli, and it is very worthwhile.

The Centovalli is a different type of scenic railway. It has much lower than some of the other routes' mountain pass. It is also the only one South of the Alps, which in this means lush vegetation and many, many small streams.

At Domodossola I boarded an Italian express train over the Simplon Pass to Brig (the train continued on to Geneva). At Brig, I boarded another train (regional) for the rest of the way to Zermatt. This is the same route that the Glacier Express uses. My train was less crowded (I saw some of the Glacier Express trains) and did not require reservations (reservations mean spending extra money to get those reservations). At Brig, there was a street fair going on and, between trains, I took time to investigate and enjoy the little town and the views of the surrounding mountains. The Simplon Pass was used by at least one of the incarnations of the Orient Express. I was able to use these trains (even through Italy) using my rail (travel) pass.