There is only one rail line between Luzern and Interlaken Ost. That train is called, wait for it, the Luzern-Interlaken Express, and it is indeed scenic. Are you going from Zurich to Interlaken? Yes, it is faster to go via Bern because the terrain is flatter, the tracks are straighter and the trains go faster, even though it is actually a longer distance traveled. If you were buying tickets rather than using a pass, the ticket via Luzern is actually cheaper than via Bern because the distance the train travels is less.
At Interlaken Ost, you change to a train to Spiez, then at Spiez you change to a train to Visp, and there you change to a train to Zermatt.
For extra credit, you can also take the "Loetschberger" regional train from Spiez to Brig and change to the train to Zermatt there. It is slower than the regular route, but much more scenic, because regular route uses a new, very long tunnel which goes completely under the mountains unlike the slow route, which is mostly over the mountain with a shorter tunnel.