Travelling by train to Grindelwald

Hello Martin,
I can think of 4 possibilities and you can check all of them through the Swiss Timetable:
1. Direct route Zürich - Bern - Interlaken - Grindelwald. It takes about 3 hours. The Alps will be visible after leaving Bern.
2. Route Zürich - Luzern - Meiringen - Interlaken - Grindelwald. This one takes about 4 hours. After leaving Luzern the views are beautiful and you will take the Brünig Pass (Altitude about 1000 m.)
3. Direct route Geneva - Bern - Interlaken - Grindelwald. About 3 3/4 hours. (See 2.)
4. Route Geneva - Brig - (Kandersteg) - Spiez - Interlaken - Grindelwald. This one will take you about 5 1/2 hours!
If your time of arrival is early enough I would suggest option 4. Your journey will take you through the valley of the river Rhône and especially after Brig it's very beautiful in my opinion. Consider to take the old route (tunnel) via Goppenstein and Kandersteg. The winding rail track between Kandersteg and Frutigen is a favourite!
Whatever your choice will be: travelling by train through Switzerland will be a special experience!
Enjoy your stay.

Update: I forgot another route:

5. Geneva - Montreux. From Montreux with the Golden Pass Line (a panoramic train - reservation required!) to Zweisimmen and from there to Spiez and Interlaken. This route is indeed very beautiful too! Check the above mentioned Swiss Timetable.