On this route, you'll have a 25% discount.

Pass validity in Switzerland:

https://www.interrail.eu/content/dam/miscellaneous/Map%20of%20Switzerland_SBB.JPG

For Swiss timetables (and prices), please use www.sbb.ch. It's much more up to date than the Interrail website or the rail planner app.

Most private railway companies in Switzerland are included in Interrail apart from the trains in the Jungfrau-region where Interrail only gives a discount. If you still have questions after having looked at the link from @rvdborgt , please ask here again.

Hi Again,

Thanks for the image, its quite clear on the routes that are free and which are having discounted. Question is how would I avail this 25% discusont having the interrail pass. I didnt find any option for the Interrail pass discount in SBB.ch

And is it free for 6 year kid?

Thanks for your reply.

You need to buy the ticket at a ticket office to get the correct discount. Please see the answer in the link below. 

https://community.eurail.com/eurail-interrail-passes-41/interlaken-to-jungfrauhoch-1468?postid=5639#post5639

I think children FROM 6 have to pay half fare. As all these trains are without any RES the age-old traditional system of just go to counter+buy ticket still applies-or maybe from machine.

There are also-in case you really specifically want these lines-local rover tickets that allow travel for a few days. A simple google for ´Jungfraubahnen´ will lead to results. Plus that sometimes even HTLs give free passes if you stay there. However, the really only train to Jungfrau-mostly in tunnel in fact-is always out of any pass-even with discount it cost €€€€€€-or rather CHF=frankeli.

On the pass you can travel on quite a few very scenic tipical Swiss mountain lines too-but not these-you may want to choose routes less overhyped.