Eurail or Swiss Pass for multi-country travel?

The Eurail Select Pass might work well for you. It can cover your choice of 4 countries and since you are going to 4 countries, you will have your bases covered for one price. Some routes in Switzerland might not be covered so you might have to pay extra for those.

Eurail Passes have gone the way of Traveler's Cheques and dinosaurs. They're overpriced, don't cover the cost of mandatory reservations, and some countries even ration the number of passholders allowed on a particular journey. Your itinerary is simple and if you purchase your train tickets early as soon they're posted you stand to get a very very good deal. Use trenitalia.com for your Italian travels, sbb.ch for your Swiss Travels and bahn.de for your German travels. Make sure you can print your tix at home for simplest use. Likewise you can choose to pick up at the train station...just make sure to keep the credit card you used to purchase them (even if it expires before you arrive).

For Switzerland you can purchase a half fare card which gives you 1/2 price tix on every trip you make. If you actually travel to the Jungfraujoch then the savings from the half fare card would probably pay for itself. Anyway, use those particular sites to purchase your tickets.

Both passes cover the Golden Pass route and both passes offer a 25% discount from Wengen to Jungfaujoch. Other coverage and discounts in the Berner Oberland are more extensive with the Swiss Pass.

I can't tell exactly the window your Swiss travel fits into; it probably is more than 4 consecutive ($223) and less than 8 consecutive days ($342), so 4 days flexi ($255) might be the best fit. The Swiss pass costs about the same as buying 2nd class tickets locally for border-Bern-Lauterbrunnen-Montreaux-border (estimate $225), but the additional transportation coverage and discounts during your stay in Switzerland would be a benefit, along with fewer stops to buy tickets.

Versus my rough estimate of $460 to buy all the main train tickets in stations as you go (to Interlaken, not Lauterbrunnen), the Eurail Select Pass costs $352 for 6 flexi days or $410 for 8 days or Global Pass is $451 for 15 consecutive days. The pass gives you hop-on convenience for most trips, but your 3 trips in Italy will require seat reservations of about $15 each. These passes also give you 25% discounts around the Berner Oberland.

Just to start a fire, if you look at other forum posts you might wish to shorten your stay in Montreux