Seat reservations are not mandatory for the Luzern-Interlaken Express, they are completely optional: Secure a seat | zentralbahn.ch. The Luzern-Interlaken Express runs hourly and is even used by the locals for commuting.
In general, seat reservations are mandatory for
- Glacier Express
- Goldenpass Prestige Class
- Gotthard Panorama Express
- (Bernina Express)
- Some international trains
On any other train, a seat reservation is completely optional (IC, ICE, EC, RJX) or not possible at all (e.g. on IR, RE, R, S, S-Bahn trains). Where it is optional, there is no often no need to reserve a seat; none of the Swiss do reserve a seat, they just get on the train and find a free seat.
To reserve a seat on SBB trains, you best use the SBB Mobile App. There, you specify the route, select the service, and select “Seat reservation” as product. You can reserve up to 5 seats per purchase. You do not have to indicate what ticket you have, as with the panoramic trains above; you simply reserve the seat.
More information on reserving a seat on an SBB train, as well as further possibilities to reserve a seat, you can find on this website: Seat reservations in trains and dining cars | SBB