Which side of the train from Bern to Interlaken do you need to sit on for the best view of the mountains?
At the left hand side
For mountains on the right, the lake on the left...
Here are the views that I enjoy, all of them depend on the right weather conditions:
Just after leaving the station in Bern - on the right you can see the Aare river, church steeples, Eiger, Mönch and Jungfrau in the background. They all line up beautifully for a short moment.
Around Münsingen or maybe Kiesen you have the Stockhorn and Gantrish mountains on the right, and will begin to get glimpses of Eiger Mönch and Jungfrau on the left. Both views have farm fields in the foreground.
Close to Thun, the Stockhorn mountain range will now be fairly close to you on the right. It is especially beautiful in the morning when the sun is hitting it.
Between Thun and Spiez you have the Niesen on the right, very close, you might even have to be at the window looking up.
Once you hit the lake, on your left, that is the best side from here on to Interlaken as you will have the Niederhorn directly across the lake with the other mountains ahead of you, further in the distance.
If you are going on to Interlaken Ost, it is once again the right hand side. The Aare river and the Jungfrau can be seen in this direction.