You can easily get a ticket by using the SBB Webshop (www.sbb.ch/en) or by using the SBB Mobile app.
The train is marked as PE and leaves Lucerne every hour, at minute xx:06.
Kindly note, the train is not fully operating between 27 Feb and 02 Apr due to engineering works. It runs between Lucerne and Meiringen. In order to get to Interlaken, you will have to change in Brünig-Hasliberg to the connecting bus replacement service. The train ticket is valid for the bus, too.