Jun 27, 2015 - 11:31 AM
There are no trains to the top of the Jungfrau, but to the Jungfraujoch, which is a saddle between two mountains. You don't need to buy tickets in advance. With a Swiss Travel Pass (it's no longer called Swiss Pass, see here) you will get free traveling to Wengen or Grindelwald, so you don't need tickets for that part. You will get a 25% discount on tickets for the remaining leg, which can be purchased at any staffed station (Bern, Interlaken, etc.). A Swiss Half Fare Card provides 50% off the whole trip.
You can click to "Where it's valid" on the Swiss Travel Pass page or Swiss Half Fare Card page for further details.