Hello, we will be traveling from Zurich to St. Moritz, taking the Glacier Express first class from St. Moritz to Zermatt, going from Zermatt to Interlaken, and back to Zurich. We want to go to Jungfraujoch and Lauterbrunnen. I know the Swiss Travel Pass doesn't cover everything, so I don't know if that's the best choice. We'll be in Switzerland about 10 days. What do you advise?
The question of which pass to buy for Switzerland is often very complicated and requires some number crunching. The ideal situation is to compare the cost of point-to-point tickets with the cost of the pass. In the past I've resorted to formatting an Excel spreadsheet to work out all the details for Swiss travel.
A couple of points to mention about the Jungfraujoch. That's an expensive trip, so before heading up there, it's a good idea to check the weather forecast and the onsite webcams, as it sometimes gets very foggy at the top. Also, the Jungfraujoch is at an altitude of about 3463 M / 11,400 feet so if you're subject to altitude-related issues, that's something to consider. I found that I was moving much more slowly than usual at the top, and climbing stairs was more of an effort.
Hopefully one of the others will have more suggestions.
Two questions: How many are in your travel group?
How many actual days will you require train tickets? It looks like 3 days plus traveling around Lauterbrunnen.
If Jungfraujoch is a must-do, the Half-Fare card makes a lot of sense, because it gives a 50% discount vs. 25% with the Swiss Travel Pass - the difference is roughly Fr. 50 in favor of the Half-Fare Card on that trip alone. With a back-of-the-envelope calculation on your overall route including Jungfraujoch and the 1st class supplement for Glacier Express, the card is very likely to come out cheaper than a 6-day second-class STP by about Fr. 100, overall.
This said, the STP is convenient: no tickets / reservation needed for Zürich-St Moritz, Zermatt-Interlaken-Lauterbrunnen, Interlaken-Zürich. And it gives the ability to do day trips on a whim, for free outside of mountain railways.
If you are looking to save money, the 1-month Half fare Card will save the most. As balso pointed out, it gives a bigger discount for the Jungfraujoch. The full fare 1st class ticket on the Glacier Express is 268 CHF plus reservation fee. But you can buy a 1st class Saver Day Pass now for as low as 49 CHF with the Half Fare Card. These go on sale 60 days out and prices vary by day, and gradually increase as you get closer to your intended travel day. Right now, beginning July 1, I do not see any more expensive than 99 CHF. Since the 1st class Half Fare price is 134 CHF, you will save at least 35 CHF per person doing that, and probably more.