One of the most popular routes for breathtaking views is the Jungfraujoch Railway, also known as the Top of Europe. The Jungfraujoch is a high-altitude mountain pass accessible by a cogwheel train that takes you through picturesque landscapes to an elevation of 3,454 meters (11,332 feet).

The train journey offers spectacular views of the Swiss Alps, including iconic peaks like the Eiger, Mönch, and Jungfrau. Once you reach the Jungfraujoch, you can enjoy panoramic views from the Sphinx Observatory and explore the Ice Palace.

With an Eurail pass, you’d get an 25% reduction for this. But it’s still quite expensive. Really expensive...

Additionally, taking a boat cruise on Lake Thun or Lake Brienz provides another beautiful perspective of the Interlaken region. Both lakes are surrounded by mountains, and a boat ride offers a serene and scenic experience. But of course this depends on the time of year. Boats at Lake Thun are also running in winter and are free with a valid Eurail pass.

Ultimately, the most popular route for views in Interlaken can depend on personal preferences and the type of scenery you find most captivating, whether it's mountains, lakes, or a combination of both.

Hi, thank you for the response, but I'm looking for the free one. You have any idea about that? Thank you.

The only free line around Interlaken is (from Bern or Brig or most popular: from Montreux -) Spiez - Interlaken - Meiringen - Luzern (named Golden Pass). 

You may find additional information here (for Montreux to Interlaken): GoldenPass Express (gpx.swiss) and here (for Interlaken to Luzern): Luzern–Interlaken Express. | zentralbahn.ch.

In spring, summer and fall, it’s free to go partly by boat (Spiez or Thun to Interlaken and Interlaken to Brienz); in winter boats are running just at Lake Thun.