My wife and I will be traveling by train from Venice to Gimmelwald in July. We will be staying in the Gimmelwald/Murren area for 7 days (yay!). We will then leave to come home via Milan. We both will have 8-Day Second-Class Swiss Travel Passes. So, I think what I need to do is buy a train ticket from Venice to Brig, and then in Brig, just show my Travel Pass to board the next train towards Lauterbrunnen (or if the train is continuing on from Brig in that direction, just stay on and show our Travel Passes to the conductor). Is that right? I would also assume that I will need to buy tickets on the return side from Brig to Milan. I've seen Rick's advice that seat reservations are probably not needed on trains, as there is plenty of room, and it's just the two of us, so we're not part of some big group. Am I missing something here, or do I have the correct understanding of how this all works?
Yes, mostly. Except:
1) The border station on that route to Milan is Domodossola. You can see that on the official Swiss Pass map here: https://www.sbb.ch/content/dam/infrastruktur/trafimage/karten/karte-sts-pass-gueltigkeit.pdf
So you will need to buy Italian tickets to/from Domodossola. If you already have your Swiss Pass and it is a through train, you just stay on the train.
2) Seat reservations are not needed in Switzerland (and nobody bothers with them unless you are a big group). But they are needed on long distance trains in Italy. Your Italian tickets will come with reserved seats. The tickets will be for a specific train, and have the seat numbers on them. Since you are already sitting in the seat you had reserved up to Domodossola, you just stay seated there.
You can buy your Italian tickets at: http://www.trenitalia.com/tcom-en
But that site will not show trains in Switzerland (it has no knowledge of Mürren). To look up times do that on the SBB (Swiss Railways) site: https://www.sbb.ch/en
Venezia S. Lucia to Mürren, note both spellings. "Murren" will not work, enter Mürren" the valid alternative spelling "Muerren"
Actually your STP covers you all the way from Domodossola which is just a little before Brig. Buy a ticket from Trenitalia.com for Venezia S. Lucio -> Milano Centrale. Also purchase Milano Centrale->Domodossola. From Domodossola your STP pass covers you all the way to Gimmelwald/Murren.
When you purchase the tickets Venezia S Lucia -> Milano Centrale your seat reservations are already included in the price. Your can choose to add a reservation on your trip Milano Centrale -> Domodossola for 4 or 5 Euro and you can choose the particular seats you want based on a map. You can choose your seats on both legs of this trip.
The sooner you purchase these 2 tickets the cheaper they will be.
Also, when we did a similar thing with the Berner Oberland Pass, the officials said the pass was good for the 'regular' train from border to Spiez, not the express, which would cost extra otherwise. It's worth asking about at the station. (I didn't notice that our train seemed slow, just not super fast, which was fine because the scenery was interesting.)