@Oris I have reviewed the matter. Correct is: You need to have an activated travel day when getting a discount here.

So the answer given below (that a travel day is not required for the discount) is not correct. The details shown in the benefitsportal linked are correct. 

You need no additional ticket for Interlaken to Grindelwald or from Interlaken to Wengen via Lauterbrunnen. 

You do get a discount for Grindelwald or Wengen to Jungfraujoch but only with an activated travel day. 

You do get a discount for Grindelwald to First but only with an activated travel day, too.

A travel day is not required for discounts.

Your pass is valid for part of the trip from Interlaken as far as Grindelwald or Lauterbrunnen so you may want to use it for that.

The pass is even valid further till Wengen (not just Lauterbrunnen) from Wengen or Grindelwald a discounted Ticket needed. 

I can offer some more tips for Interlaken - consider taking a boat ride on Lake Thun:

You can enjoy a free boat ride on Lake Thun, which you can access with your Interrail pass. For information about the boat schedule, you can visit the following website:

https://www.bls-schiff.ch/de/schifffahrt/fahrplan

 Although the information on the website is primarily in German, you can easily find the timetable in PDF format. Additionally, within the PDF, you'll find the information available in English as well.

https://www.interlaken.ch/erlebnisse/poi/bls-winterschiff-thunersee

Just had discussion face to face with the officer in the station as well as email with info@jungfrau.ch

All sources still says it requires travel day to get the discount, even only from eigergleitscher jungraujoch.

Hope this helps

Wordings from the email. 

"From Grindelwald or Wengen further up to the Top you receive a 25% reduction on the normal fare to Jungfraujoch and return."

"Yes, you have to activate the pass to get the discount"

This is incorrect: to make use of the discount, you don’t need a travel day. If they make you use one, then ask customer service to give it back. If they make you pay extra, then claim it back from customer service:

https://eurail.zendesk.com/hc/en-001/requests/new

@Mukhammad @Camilo. Could you please follow this up with the Jungfraubahnen? We've seen a report about this very issue before on this community. It looks like their employees have been provided incorrect information about how the pass works with a discount.

A travel day is never required to get a discount less than 100%. Only if the discount is 100%, that is when you either don't pay anything or if you only pay for the seat reservation, you must use a travel day. All other information is wrong so you have been misinformed. 

See information from the Interrail benefit page below.

Might be a terminology issue too. You don't need a travel day but it must be within the validity period (1-2-3 months).

Maybe. I'd be curious to see the e-mail from the Jungfraubahnen.

An active pass is not the same as the use of a travel day. An active pass means that your pass is valid on the date you use the discount.

Thanks, I overlooked that quote.

Indeed, you do need an active pass.

@Oris did any staff insist on an active travel day or only on an active pass?

Asked 1 email, 1 ticket officer in station, and 1 ticket man in train from eigergleitscher to jungfraujoch.

They all says the same. I ask personally and twice, also differentiates the terms of validity(which 1 month), and travel day.

They all says the same. We need to activate travel date. Interlaken ost to grindelwald, than all good for the rest of leg. 

I overheard persons accross me, they didn't activate travel day, ticket man asks them to pay 25chf each person in Place. 

Thanks for the additional information.

@Camilo. @Mukhammad This is really reason for concern. Jungfraubahnen are still not applying the rules correctly.

This is correct though. You need a valid travel day for Interlaken Ost - Grindelwald as this bit is fully included in the pass.

Agree for this one

 But while in the train from eigergleitscher to jungfrau, the ticket man keep asking for active travel day. To get the discount 

But in that case the question is how the traveller got to Eigergletscher without an active travel day? On a normal ticket?

As written before: You do need an active travel day to get a discount. 

This time it's even correct in the benefitsportal: https://benefitsportal.eurail.com/benefits/jungfraubahn/.

JB demands an activated travel day for the reduction. That's their decision. 

That means they want to have it both ways: income from tickets as well as income from the pass.

I didn't know that was even a possibility. According to the conditions of use, that is in fact not possible. Under "Pass benefits”, it says:

For holders of a Flexi Pass reduction benefits can be used during the entire period of validity of the Pass. Free travel benefits for Flexi Pass holders can only occur on a travel day.

That clearly means that you currently don't need an active travel day, regardless of what is stated on any other pages or what JB claims.

@Mukhammad @Camilo. I thought this was being cleared up with Jungfraubahnen and SBB. However, I can't find that comment back anymore. Anyway, what's the status? Operators are not allowed to require an active travel day when you travel on a discount, so they can't keep on handing out fines and/or forcing people to use a travel day.

Dear all, 

I will investigate this with the relevant SME. Considering that our Benefits Representative recently transferred to a different role, this could take some time. I will keep you updated. 

Have a nice week!

Did this trip yesterday from Lauterbrunnen.  When planning our journey some time ago no travel day was needed so we didn’t activate one.  On the train we were told that this was needed but conductor would not give us any assistance in activating day without adding a journey.  She said we either had to pay the discount (€228 for the four of us) or get off the train.  We paid.  On the way down we stopped at Wengen and a more helpful man at the travel office helped us to activate a day and gave us a full refund.

This is our first Interrail trip which was going smoothly until yesterday and it has definitely left a sour taste in the mouth.  The relevant sites really need to get their act together to make the rules more clear. There were at least five groups of travellers in our carriage in the same position.  One couple leaving the train.

We are heading to Lucerne now.  Does anyone know if the rules for Mount Rigi changed as well

The rules are clear: no travel day is needed when you only get a discount. This is clear from the pass conditions of use. The Jungfraubahnen know this but they choose to ignore this. If you only travelled on sections where you had a discount, then contact customer service to ask your travel day back.

@Mukhammad @Camilo. What is the status with Jungfraubahnen? They are now making multiple victims every day. IMO this is getting out of hand and needs to be solved ASAP.

Thanks for your response.  I have contacted customer services.

After a number of emails with customer services an additional travel day has been added to our passes and they have confirmed - ‘In the Jungfraujoch route (Jungfraujoch – Kleine Scheidegg) there's no need to activate a travel day, but you do get 25% discount as a Pass holder.’.

Thanks rvdborgt for your response. I feel for the many other people who have repaid the 25% discount and the people who got off the train.