<<"Sorry for the confusion. We are flying from Nice to Zurich to start our rail tour - our daughter will be meeting us in Zurich coming up from Venice. Ultimately we fly back to the US out of Venice on the16th">>
Lets start at the last part, first.
You may use the timetable and a good Swiss map to understand these routes:
or the direct link without the useful explanations to the same timetable if you are not in My Swiss Alps:
Lots of detail buried in the menus.
Also with lots of useful travel info in the menus. For example, you can select "Railway" under Points of Interest on the menu, then Traffic, and all the rail stations show up. Zoom in until you get icons and click on a station icon, and an abbreviated timetable shows up.
There are three commonly used rail routes between Switzerland and Italy. As far as i know, you'll go through Milano Centrale on your way to Venice.
I used to periodically take the Gotthard Pass route from the southern tip of Lake Lucern (Flüelen) to Altdorf thence to Bellinzona and beyond , usually to Locarno, but sometimes to Lugano and once or twice to Chiasso and beyond to Milano Centrale, or further to Florence. And return. To get to Flüelen, I would start at either Zürich or Luzern, both about the same distance from Flüelen. Either from Luzern or from Zürich, the routes pass through Arth-Goldau which is major junction/ crossover points for the major north-south rail axis.
If you are at Luzern and have lots of time (3-4 hours) , you can ride a lake boat from Luzern to Flüelen and then go onward to Lugano, Milano, Locarno, etc. by rail; you may use the named route "Wilhelm Tell Express" or do it your self in stages, following exactly the same route but at times of your choice.
Train from Luzern to Füelen takes about an hour.
The Gotthard Pass rail line is a major engineering masterpiece, with superb mountain scenery. It carries a tremendous amount of rail traffic, hence a new lower 57 km long "basis tunnel" will open later this year, to absorb the traffic. Views of the Alps and the engineering works are absorbing and make for a very scenic ride.
From Bern or Interlaken via Spiez, then over or under ( don't go under in the boring but faster new basis tunnel) the Lötschberg Pass, into the Rhonetal ( Rhone Valley) and down to Brig.
From Brig over/the Simplon Pass (including tunnel) to Domodossola in Italy. From thence to Milano Centrale. Northbound, the route splits at Brig and takes you to Geneva via Montreux or to Spiez.
( I rode this route many times between our office in Milano and our laboratory in Geneva.)
You an also pick up the scenic narrow gauge line to Andermatt and eastward, or thence to Göschenen on the Gotthard line. That is part of the route of the Glacier Express. It is not a fast route to Italy.
The section from Spiez to Brig via Kandersteg ( us that as a "via" or "connection" in the timetable) on the train "Lötschberger" is very scenic. I have ridden it many times and hiked the "rail trail" alongside the rail line above Brig many times.
From Milano Centrale to Domodossola the rail line has some nice views of the lakes as well.
I rode this route many times between our office in Milano and our laboratory in Geneva.
A limited service and very special rail journey is the narrow gauge Centovalli line between Locarno and Domodossola. Not fast.
#3- Part or all of the Bernina Express route, specifically the part from Pontresina ( St.Moritz) to Tirano in Italy, then bus to Lugano thence southward, or train from Tirano to Milano Centrale. From St. Moritz to Milano Centrale - just under 5 hours.
That section of the Bernina route between Pontresina and Tirano is also very scenic.
Getting to St. Moritz can be quite scenic as well, but does add considerable time on the route from central Switzerland.
Those are some ideas for routes to finish the trip.