There are multiple routes that can be taken for Lausanne - Interlaken.Via Bern is the quickest but others with travel on very scenic lines can be made also. All are valid with passes so wherever you are getting information from is incorrect.

Where do you get the info that the pass is not valid? 

Going from Interlaken to the Jungfrau-region the pass only gives a 25% discount.

@AnnaB @Al_G Thank you for replying quickly.

I am using the app Rail Planner, and it looks like that on weekend days the trip from Spiez to Interlaken is not covered by the pass

It could be that there are engineering works. When in doubt, always check the (national) operator’s website. The Rail Planner App doesn’t update very often.

Did you check: https://www.sbb.ch/en/timetable.html ?

Actually the journey exists between Lausanne and Interlaken, however the last connection between Spiez and Interlaken is not covered by Interrail Pass cost, on saturdays and sundays. That how it looks like to me. I did the sumulation on week days and it is entirely covered with the Pass.

In that case the Railplanner is wrong. The trains between Spiez and Interlaken West/Interlaken Ost are all included in the pass.

What dates are you looking at? 

@AnnaB I am looking at a trip on saturday 05/11 from Lausanne to Interlaken Ost/West

There is a replacement bus on which your pass us valid, no matter what the Railplanner says.

Perhaps!! There is a rail-replacing and there is ALSO a regular normal (former trolleybus) busline along (I think its line 131) parts of the lake, more intended for pax going to intermediate places-so you have to look carefully which one to use. There will be staff and just ask.

Indeed-beyond INterlaken-toward the mountains, the pass wil only give discount on what are even with d quite high fares. The Zentralbahn line to Luzern is however free/included in pass.

When you go from Lausanne to Interlaken I would recommend you to go the Golden Pass route from Lausanne to Montreux and via Gstaad to Zweisimmen and Spiez. It will take 2 hours more but you will have a nice views. First along Lake Geneva from Lausanne to Montreux, where you change trains, sit on the right side of the train. And then from Montreux to Zweisimmen, where you change trains again. And finally from Zweisimmen to Spiez and on to Interlaken.

Apparently there are also construction works on the first part of the route from Montreux to Zweisimmen but there is a replacement bus available.

Thank you for your replies, they’re very helpful @AnnaB and @mcadv 

The problem here seems to be that the rail planner app contains a timetable in which the trains Spiez - Interlaken have already been cancelled on 5 and 6 November, but the replacement buses are not included yet. So it shows a regular bus service Spiez - Interlaken on which rail passes are not valid.

In the mean time, SBB have published the timetable of the replacement buses, but the rail planner app won't know about them until its next update (or the one after that).

Pease don't use the rail planner app to plan journeys. It's not fit for that purpose.