Surprise: jungfrau.ch Dr google sent me there for more info.

Says: these passes currently ONLY available at counters locally. Just as EUrail passes they cannot sell out AFAIK-but perhaps some trains on populair timings can get full. Price for 3 D 229 CHF-or it also lists 179 ´reduced´ but there is-or I could not find it quickly- no easy listing on to whom should be entitled to reduced-perhaps showing your pass could do the trick-or not.

Todays craze: anyone seems to believe that one has to handle all these things online and preferably on the cherished fone- have all forgotten how these things worked not really that long ago.

The Top of Europe Pass is not possible to combine with Eurail as far as I know.

With Eurail you get 25% discount on the normal tickets in the Jungfrau-region. Those ticket you need to buy at a ticket office in Switzerland. 

Don’t worry. You buy the ticket at the ticket offices in the area. No reductions for Eurail. When you go try to be on the first services and use the lift from Grindelwald. You might be able to book a specific departure from Eigerglacier again from a ticket offices. There can be very long waits to get up to Jungfrau if you aren’t there early and don’t reserve! Choose your day carefully and check the cctv and LOCAL weather reports. Conditions change very rapidly on Jungfrau. It’s a brilliant trip but the most expensive in Europe!!

You can reserve seats up to Jungfrau online as well as at the ticket offices 10CHF each seat. The only discount on the 3 day pass is for holders of a Swiss Half Fare pass or a Swiss GA. Don’t buy this pass if you intend to take skis or a snowboard with you. You will need to purchase the correct area ski pass for this.