day trip from Interloken to Jungfrau and or Grindelwald

There are regular (every 30 minutes) trains from Interlaken Ost to Grindelwald, trip takes 34 minutes.
You can look this all up on the Swiss Railways website: http://www.sbb.ch/en/home.html

From Grindelwald you can take a mountain train up to Kleine Scheidegg, or other cable cars up the side of the valley.
A Eurail pass IS NOT valid past Interlaken, you will have to buy a ticket but you will get a 25% discount. Details here: http://www.eurail.com/europe-by-train/switzerland/swiss-railway-companiesa

If you're able to move your accommodation plans from Interlaken, you really want to stay in Lauterbrunnen or even better would be Murren or Wengen. The views are incredible in those towns!

You can easily start your journey to the Jungfraujoch (Top of Europe site) from Interlaken as mentioned but also as already mentioned starting in the village of Lauterbrunnen offers a 15-20 minute head start on visiting either side of the valley by train or cable car.

Don't overlook taking the cable cars to the Schilthorn - if you only have time for visiting one side of the valley - Jungfraujoch or Schilthorn - before you start you should check the forecast and the live webcams for your destination to make sure that the weather makes the trip worthwhile. Nothing like paying $400 (2 people) to get to the Jungfraujoch only to find out that is was totally cloud covered for the whole time you were there..(this happened to folks that were staying in the same B&B as we were). Meanwhile we had gone up to the Schilthorn after checking the cameras and had great views.

And while the sights from the peaks are fantastic - the views from the villages along the lower edges of the valley walls like Mürren, Gimmelwald and Wengen are stunning also.

Have fun!

Your Eurail pass is not completely useless past Interlakken. While you will not be able to ride for free, you will get a discount on all mountain railway tickets if you show them your pass. I must agree with all of the posters that said if you can move closer to the mountains to do so.

Thanks for all the replies! But I'm still confused as to why my Eurail Pass is invalid "past Interloken" when the tickets are through the SBB website. This is listed as covered on my rail pass... I get that from Eigerletscher to Jungfrau is extra, but perhaps I get a discount by purchasing the Good Morning ticket with the Eurail Pass. Whew!

Skytribe, The Eurail pass is not a ticket from the SBB. AFAIK SBB does not sell Eurail passes (I can't find any reference to it on their website).
Eurail is an organisation operating from the USA, created just to sell these passes, mainly to the North American market. It's main selling point is it allegedly makes travel "easier" for North American visitors who then do not have to buy tickets for each journey. It then pays money to the railway companies to carry the passengers with the pass, the companies can of course choose to accept or not to accept this offer.
The line from Interlaken to Grindelwald is not owned by SBB, it is owned by the BOB (Berner Oberland Bahn), which presumably has turned down Eurail's offer.
On the Eurail website (click here: http://www.eurail.com/europe-by-train/switzerland/swiss-railway-companies ) it says:

BOB Berner Oberland - Bahnen
Routes: Interlaken Ost – Wilderswil – Zweilütschinen – Lauterbrunnen Interlaken Ost – Wilderswil – Zweilütschinen – Lütschental – Burglauenen – Schwendi – Grindelwald
Discount: 25%

SBB sell a "Swiss pass" which is valid past Interlaken (note spelling).

BTW, the line to Interlaken is not SBB either, it is BLS.

sky,

I didn't see this post until after replying to your other one, so wasn't aware you'd be staying in Interlaken. Which two countries is your pass valid for?

I assume you're referring to Interlaken? One reason your pass may not cover transportation from there is that you'll be using different (private) railway networks and mountain lifts as you travel from Interlaken. The first one will be the Berner Oberlandbahn which goes from Interlaken to Lauterbrunnen. When you arrive in Lautebrunnen, you'll transfer to the Wengernalp bahn rack railway for the trip to Kleine Scheidegg.

You'll use a variety of rail and cable car systems, depending on where you're going. These are all well integrated, with typical Swiss efficiency. As someone else suggested, you might also consider going to the Schilthorn rather than the Jungfraujoch, as it's a bit cheaper.

Once you've made a decision on which mountain to visit, the group here can help with more specific directions.

A 2-country pass is a Eurail pass, not a Swiss Pass---even if one of the two countries is Switzerland. So it is subject to the restrictions on all Eurail passes.

You can see the area of full and discounted validity on the Eurail map given to you previously in another thread.

The discount past Interlaken, all the way to the top of the Jungfaujoch, is 25%. If you were to buy your ticket on jungfrau.ch, you will see that the full price is 210.80 for a roundtrip from Interlaken and back. If you click on the drop-down menu for "reduction" ( it is right after the birthdate entry), you will see a listing for Eurail Pass. Click on that and the price drops to 158.80---a 25% reduction.

But you would be better off buying it there if the weather is cooperative, as suggested above.