Currently only the GoldenPass Panoramic and GoldenPass Belle-Epoque trains between Montreux and Zweisimmen as well as the GoldenPass Express trains between Montreux and Interlaken Ost (from the end of 2022) can be booked online.
The Luzern-Interlaken Express trains between Interlaken Ost and Lucerne can be booked directly via the Zentralbahn.