April to October is peak season on the Lucerne-Interlaken-Express.
In order to take the load off high-demand trains, the operator, Zentralbahn, offers additional services from Lucerne to Interlaken and back from April to October.
Additional trains
The timetable for the additional trains can be found here:
→ https://www.zentralbahn.ch/en/experience/leisure/luzern-interlaken-express/luzern-interlaken-express-s-lix
Seat reservation
The Zentralbahn strongly advises you to reserve a seat on the Lucerne-Interlaken Express, especially for the trains at leaving at 9:06 a.m., 10:06 a.m., and 11:06 a.m. from Lucerne.
→ Link to the seat reservation
Also take note of the baggage space issue:
https://www.zentralbahn.ch/en/get-to-know-us/zentralbahn/insights/luggage-situation-on-the-luzern-interlaken-express
All information without guarantee.