Hello 2025Visitor
Welcome to the SBB Community.
I suggest that you should look at the rail network map:
https://maps.trafimage.ch/ch.sbb.netzkarte?baselayers=ch.sbb.netzkarte,ch.sbb.netzkarte.dark,ch.sbb.netzkarte.luftbild.group,ch.sbb.netzkarte.landeskarte,ch.sbb.netzkarte.landeskarte.grau&layers=ch.sbb.geschosse2D&lang=en&x=874431.94&y=5850740.4&z=10.34
You will notice that Zermatt-Brig-Kandersteg-Spiez-Zürich is not the most direct route. It is the old route, before the Lötschberg basetunnel was opened in 2009. The direct route now goes Zermatt-Visp (change) Visp-Spiez, trough the Lötschberg basetunnel.
What you want to see is the now called (old) panoramic route over the mountains. For this, you have to travel from Zermatt to Brig (change), Brig-Bern or Brig-Spiez (change) Bern or Spiez-Zürich.
You have to work with Via’s (Settings) in the timetable query:

The fastest connection is 4h 22min.
When you travel from Interlaken, you probably will take the IC line via the basetunnel, to Visp. So no, you will travel two different routes between Visp and Frutigen.
Kind regards.
Roland