No, trains from Interlaken Ost to Lauterbrunnen, Grindelwald and above are not covered by Eurail. You get a 25% discount though : tickets available at all Swiss ticket counters (and maybe at Interlaken Ost ticket machines, I'm not sure).

The 25% discount also applies to the Grütschalp cable car or the one to Schilthorn.

You can see times and prices from Interlaken to Lauterbrunnen etc on the Swiss railways website, sbb.ch.

It will often default to showing you the half fare price, so make sure to switch it to full price. You can then take 25% off the full price to see what you’d pay with the Eurail discount.

 You can even buy the ticket (with discount) from the conductor onboard your train to Interlaken to save time

Are you sure this is still the case ? It may be doable if the conductor is nice enough but they don't have to. The fine for travelling without a ticket is 90-100 CHF + ticket price.

Source : https://mobile.jungfrauzeitung.ch/artikel/172544/

I was able to buy an Interlaken-Jungfraujoch ticket onboard the Intercity from Zurich to Interlaken. Conductor offered me the Interrail discount before I even asked.

I wouldn’t board any Swiss train without a tickets! You can always buy at the ticket office