7 days Zurich, Interlaken & Montreux in Sept

Hello Tina! Zurich Airport has its own railway station, from which you can easily travel to all directions. You can plan your trips by train and bus through the online timetable. For example: fill out Zurich Airport as your arrival station and Lucerne as your destination. The timetable will then come up with all available connections. You can specify dates and travelling times as well. This also goes for your trip from Montreux to Milan. You can enter both stations in the timetable and check the possibilities. The easiest connection will be a EuroCity train (for which seat reservation is required), which runs directly between both towns and doesn't require a stop over.

From each town in your itinerary you'll be able to enjoy beautiful views without having to make gondola trips. If mountain trains are less of a problem for you, there are quite some options. From Lucerne you can make boat rides over the lake and take the mountain train up to Rigi, which is a wonderful viewing point. The train trip is very relaxing and doesn't include deep ravines or any other spectacular passages. The view up there at Rigi is spectacular though. You could also get up to Pilatus by train from Alpnachstad, although this ride is rather steep at some points. Try to explore Lucerne's city centre and the famous Kappel bridge as well.
As Interlaken is situated between lake Thun and lake Brienz, boat trips on these lakes are a great day trip as well. Mountain trains in this area are available, too. You may consider the cog railway trips op to the Brienzer Rothorn or Schynige Platte, and the most famous attraction here - the Jungfraujoch - can be reached by train as well. Large parts of the route run through a tunnel in the mountains, so you won't be confronted with heights that much.
Montreux, along the shores of lake Geneva, offers the option of boat trips as well. You may want to visit the Chillon castle, and a trip to Mont Blanc can also be made within one day. You'll first have to travel to Chamonix by train and then pick one of the cable cars or mountain trains.

As for a rail pass: as you indicate that you'll be travelling almost each day and as you may want to use regular public transport (trains, buses and boats) more than mountain transportation, an 8 day Swiss Pass will probably be best in your case. The link 'Overview of the network on which the Swiss Pass is valid' allows you to explore the exact area of validity. All regular train, bus and boat trips will be for free. The Rigi train ride is for free as well. Other mountain trips are reduced by either 50% or 25%.

I hope this helps!