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To save me missing the question when you want details on the train trip, let me answer it now.
You want a ticket from Luzern to Wengen via Brünig (or via Meiringen, gives the same result). You don't need different tickets for each bit in Switzerland, just the beginning and end points.
Unless you have a restricted ticket for some reason - unlikely - your ticket will be valid all day and not restricted to specific trains, just going in a straight line without backtracking.
There are three segments to this particular journey.
First part - Luzern to Interlaken Ost. Your train will probably leave from platform 13 because it needs to be in the small narrow gauge area of the station. Luzern is a dead end station so the platforms are side by side, so finding the right one is easy. The entry of each platform is at what is the back of the train. The best views are on the right hand side of the train (if facing the front) although both sides are pretty good. The train will go up and over the pass, using the cogwheel at some times and not at others. A few stops along the way, great views. After the train comes down from the pass it runs across the flat land to Meiringen (where meringue was invented and where Sherlock Holmes had his battle with the deadly professor at Reichenbach Falls - there's an ancient funicular up there) where it stops for a few minutes and changes ends. So if you were facing forward you'll be backwards and so on. But you'll be on the best side again as you reach Brienz and the lake named for it and the Giessbach Falls across the lake.
You will cross the lake at the other end and arrive at Interlaken Ost station.
Go to the platform just off to the side for the train to Lauterbrunnen and Grindelwald. Half of the train goes to one and the other half to the other. Don't get on the wrong half - they are clearly marked - or you will wind up at the wrong destination. It is written on every door. Sounds worse than it is, You want the one for Lauterbrunnen. Stay on until the end. When everybody gets off, get off.
Just across the Lauterbrunnen station will be the green and yellow (usually) train to Wengen. It is also well signposted and there will be announcements in several languages including English.
Hop onto the Wengen train for the short but beautiful journey up to Wengen....
Job done...