Train options for Venice to Lauterbrunnen - will have Swiss Pass

this is a duplicate post - its stable mate is over in Italy.

The Venice > Milano bit will be pretty much the same regardless of the Swiss bit. High speed Trenitalia Freccia Venezia Santa Lucia > Milano Centrale.
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In Milano Centrale you will change onto a Eurocity (EC) train of probably Swiss carriages which will take one of two routes.

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(A) The more eastern route will go Milano> Gotthard Pass > Arth Goldau >> change for Luzern > Luzern Hbf >> change to the narrow gauge ZentralBahn > Brunig Pass > Lake Brienz > Interlaken Ost.
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(B) The more western route is via the new basistunnel, Milano> Domodossola > Brig > Basistunnel > Spiez >> change for Interlaken > Interlaken Ost.

There are also scenic trains through the mountains like the trains between Chur and Tirano but they will take most of the day by themselves and when added to the travel from Venice and the further travel into Interlaken afterwards are not a reasonable journey in a day.

So, to your 3 questions:

1) B....
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2) A. Not only do you wind around Andermatt, seeing the same church 3 times from different views, in dogbone tunnels and loops in the mountains, you are on a rack railway going over the very scenic Brunig pass and into Meiringen where meringue was invented and where Sherlock Holmes fought his nemesis at the real Reichenbach Falls (you can climb them when not on the train) just out of town. The engine changes ends and the front is back and the back front. You then follow the exceedingly beautiful Brienzersee into Interlaken Ost.
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3) The Italian bit will be the same, and you don't pay extra beyond the pass - so neither.

Heidi, you don't say when you will be traveling. The quickest and least expensive way is to buy a ticket online from Venezia-Santa Lucia to Spiez, Switzerland. The Italian train system (Trenitalia) cannot see Lauterbrunnen on their system. I can see advance tickets 90 days out right now for as low as 31Euro (9Euro Venice/Milano and 22Euro Milano to Spiez). The tickets are non-refundable. I don't think you can beat this fare anywhere.

At Spiez, you are a 20 minute ride away from Interlaken Ost. First, go around the back of the Spiez train station and use the ATM there (if it's still there) to withdraw Swiss Francs. Second, go into the station and buy your tickets (with your pass) to Lauterbrunnen. You can then either use your Pass, new Swiss Francs or your credit card (not sure which pass you have). About 6.0hrs for this part of the journey. For Spiez to Lauterbrunnen, you are about an hour away.