Just out of curiosity, there are more than two second-class compartments on Luzern–Interlaken Express, but if I try to book a seat through the Zentralbahn website (https://store.luzern.com/luzern_en/specials/daytrips/zentralbahn-int-lu), only compartments nos.3, 4 and 7 are available for booking.
Compartment no.4, which is labeled as the second class, doesn’t have a seat available for almost every train, so in fact, only compartment no.3 is available for booking a second class seat. I’m looking for a compartment that offers luggage space, but unfortunately, it seems like there is no room for large suitcases in compartment no.3.😕