Even though you now start the purchase of a train ticket by selecting a train that does not mean that your ticket is for that train only. Tickets are by default for a route, not a train, and it is in order to determine what route you wish to travel that you select a train.
In the past when buying train tickets one would only enter origin and destination, and the date.
Note that you do not need reservations on the Luzern - Interlaken train. That is just a normal standard service, used by locals to go about their daily business. The reservations are a rip-off in my opinion.