Interlaken to Luzern

The most scenic route would be the Golden Pass train direct from Luzern to Interlaken Ost station. There are departures every hour. Check schedules here: http://rail.ch

Yes, a beautiful ride (did it week before last). You'll want to confirm this, but I think i) only some of the trains are designated "Golden Pass" and ii) it doesn't matter since they all take the same route.

I did that route, from Luzerne to Interlaken and return, and it is, as mentioned, a very scenic trip. I departed on the first train of the morning( 7:00 am I think it departed) and returned on a late train( depart Interlaken around 8:00 pm ). I found both trips great as the light at those times of day is softer, and gives that little extra oomph to already beautiful scenery.
The only issue I had, and perhaps you will as well, is that taking pictures through a train window is less than ideal, and I have plenty that show window glare...but since your start and endpoints( Interlaken and Luzern) are both sceneic, I wouldn't worry too much, plenty of opportunity to get nice shots off the the train..

If you do take the Golden Pass, the best car, called the Panorama, with windows all the way to the top, is one of the first class cars I believe. The local attendant told me a reservation is not required, but if you want to ensure you are in the Panorama car, for the views, then consider getting one. When I took this last year, it departed Interlaken at 10:04 am and arrived Luzern exactly 2 hours later. I think there was another train going from same origin to same destination, but along a different route, with a different transit time, so check it closely. And yes, you will get glare this time of day because the sun is directly overhead, to the south where many of the best lake/mountain views are. My take ... just enjoy the views and the memories, rather than trying to take so many pictures, which sometimes precludes one from actually "seeing" the view firsthand.