In country info of the passes you can find a long list of which train companies in the Swiss accept passes.

Exceptions are most of these short lines that go up the mountains-but most give a discount still-AND the BOB=the trains south of Interlaken-to Grindelwald-Wengen (its with e -with a is in Germany).

There is yet another ALTernative-but only if you plan well advance-order Swiss cheap Daytickets via SBB-same valid as Swiss Pass (incl all local transit and the yellow postbuses too), these are -if you do not need more as 3 -4 cheaper as Swiss Tourist pass, but ONLY valid from 9.00 on mo-fr.

Note that its easy to check prices on the swiss planner-sbb.com, BUT these are HALF price-for those who already have the HalbTaxKarte-normal/foreign people pay double! So that could make a disastrous mis-calculate. These mountain lines are always outside of all passes-except then the discount.

With Interrail you only get 25% discount on the train ride from Interlaken and into the Jungfrauregion. The train rides there are probably the most expensive ones in Switzerland.

In my opinion the Jungfrauregion with Grindelwald, Lauterbrunnen and Wengen are mostly touristic and expensive. There are many other nice, less touristic and very nice places to visit. The Gelmerbahn, the trip to Montreux over the Golden Pass and also Gruyère are well worth a visit.

Here you can read about some scenic train rides which all can be made without reservation if you have an Interrail pass.

https://www.swissactivities.com/en-ch/panoramic-trains-in-switzerland/

This maps shows Interrail validity for Switzerland (including lines with a discount only):

https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-geltungsbereich-InterRail-EuRail.pdf

This map shows the validity of the Swiss Travel Pass (including lines with a discount only):

https://www.swiss-pass.ch/wp-content/uploads/pdf/swiss-travel-system-map.pdf

Thank you so much for your reply.

About the mis-calculation: the calculation I did is with Omio, a site/app that shows trains and busses, and it’s possible to buy the train tickets online on it. I just checked now on SBB site and yes, the price on Omio is double! for exemple on SBB price from Interlaken to Lauterbrunnen is 4,80 CHF, while on Omio is 12 euros… but this is not expensive to me, why is everyone saying these ticket in Lauterbrunen/Grindelwald area are expensive? I was starting to be worried expecting something like 50 euro per ticket..I don’t get it… so it should be no problem if I buy these tickets on Omio, no?

The only thing now that I’m checking in SBB site, is that, if I put Interlaken west bahnhof to Lauterbrunnen for example, it’s telling me I have first to take a bus to reach the track of the train..it’s included in the price of the ticket right?

The daytickets alternative sounds good but I wonder if it’s the one I already heard about, because someone told me one day ticket is more than 90 CHF.. and that’s a lot. But probably you are talking about something else? anyway I will need also saturday and sunday, so maybe it’s not convenient for me..

keep in mind that the price on sbb.ch is for people with an Halbtax Card, this gibes you 50% discount in Switzerland. You will probably not have this card, so you have to pay the full price.

The prices on sbb.ch are including all transportation. Sometimes on less bussy trains you can get discount tickets, they are only valid on the train you have booked.

The train for Lauterbrunnen leaves from Interlaken Ost. You can walk from Interlaken West to Interlaken Ost if you like. The distances is about 2 kilometres. 

The expensive trains are those going to up to Jungfraujoch, which is over 100 CHF one way from Interlaken and Schilthorn, which is over 70 CHF one way from Interlaken. 

Ok, I’m not going to Jungfraujoch anyway, so good, I don’t have to worry about that. But about going from interlaken to Lauterbrunnen or Grindelwald, in the Omio app is telling me the train is from Interlaken West… do you maybe know why?? maybe it’s also a bus ticket included? or maybe they are two train so I’m switching at some point https://www.omio.com/app/search-frontend/results/LBF035304998A44C9ACBFC4F8FEB4F325/train?locale=en

anyway I will stay in Unterseen, so to reach west is 30 min and ost is 40 min, how much is usually bus ticket for tourist? I can walk but maybe 40 min is too much.. It depend on this I can understand if buying interrail is actually convinient for me or not

Because for now I think my options are or to buy everything without interrail, or to do interrail and it will cover everything (bus also?) except for lauterbrunnen and grindelwald that will just have 25% discount...right?

It looks like Omio replaces the non-specific "Interlaken” with the station Interlaken West and then proposes a journey with a change in Interlaken Ost.

For Switzerland, just don’t use Omio. Use the SBB website or app.

Please check prices on sbb.ch. It contains all public transport in Switzerland so you can request a journey from/to Unterseen. Just make sure to check the full fare (and not the default half fare it shows).

Interrail generally does not cover bus journeys (in Switzerland only a few are included). Check the map I've shared above for what Interrail covers in Switzerland.

I’ve checked the map, it’s just a bit complicated for me… but at least if I understood correctly, Bern to Interlaken to Lucerne to Basel are covered by interrail right? so I would just have to add Lauterbrunnen and Grindelwald 25% discounted ..

Yes, exactly. That ticket you can buy at the station in Switzerland (with the discount)