The Golden Pass Express train does not have mandatory reservation. That train is used by locals to get about their daily business after all.
Contrary to what is mentioned at the start of this thread it is not so that "PE" trains require reservations. In the past the PE from Interlaken to Luzern was just branded as an IR. A few years ago someone at the railways must have realised that they could get more money out of tourists by branding certain trains as PE.
What you should look for is an R in a square. The symbol R means "reservation is possible" and R with a square around it means "reservation is compulsory". Those symbols are used by other railways too btw. You will also find that on www.bahn.de for example.
Reservations are also separate from tickets. That is because ticket revenue is shared by all public transport operations in Switzerland, whereas the reservation charge goes to whoever operates the vehicle.
Only the following trains require reservations:
- Glacier Express.
- Gotthard Panorama Express. (But not the Treno Gottardo, do not confuse those...)
- Bernina Express. But only in the panoramic cars. There are always several normal cars at the front of the train that do not require reservation.
- On the Golden Pass route reservations are not required, except for the excellence class seats on the GPX. They are in my opinion a waste of money on Interlaken - Luzern, but in summer I would advise them on Interlaken - Montreux.
Reservations on PE trains must be done directly with the operator. You can usually not do that via the SBB system. But there will be a link to the website where you can reserver mentioned with the train details.
Anyway: Reserve Glacier and Bernina express on www.rhb.ch and Golden Pass on www.gpx.ch (for Montreux - Interlaken) and www.zentralbahn.ch (for Interlaken - Luzern. But for that part I would not bother).
For Gotthard Panorama Express go here: https://www.gotthard-panorama-express.ch/en/