A warning about the train between Interlaken and Lucerne, since I just rode it a week and a half ago: This train can get very crowded, especially (it seems) in the mornings and late afternoons, which are the times you'd want to take it for a day trip. If you're not among the very first people to board the train, you might not get four seats together for your group. It is possible to pay to reserve a seat; however, on the train I was on, I witnessed multiple confrontations between people who claimed that others were sitting in their reserved seats. (There were electronic indicators at each seat that were supposed to show whether the seat was reserved, but all the indicators in the car I was in showed nothing. I never did figure out whether the indicators just weren't working, or whether the people were looking for their reserved seats in the wrong car, or what. Either way, it was a chaotic experience that made it harder to enjoy the train ride.)
Part of the draw of this particular train is supposed to be the "semi-panoramic" train cars. What this means is that there are small windows in the ceiling of the train, above the overhead luggage racks. Even if there were any scenery worth seeing up that high (there's not), you wouldn't get a good view of it, because there'd be luggage in the way. All that the panoramic windows do is allow more sun to enter the car, which creates more glare.