Fastest route to Lugano

The fastest route is via Luzern, which is also reached with the Berner Oberland Pass. Travel time is 5 hours, with two changes, in Interlaken and Luzern. This train will use the Gotthard Basistunnel so you will not see much scenery after Luzern, but the ride between G-Wald and Luzern is very scenic.

I did not check the travel time going via Brig to see how much longer it would be, but I will note that this route also uses long tunnels. It would be scenic as far as Kandersteg, then you enter the Lötschberg tunnel and emerge above Brig for the trip down the mountainside. Between Brig and Domodossola you are in the Simpln tunnel under the Alps. Note that the Berner Oberland Pass only covers the "old" route to Brig, not the faster one using the Lötschberg Basistunnel. You would need to be careful to board the right train. Also when checking schedules and travel time on this route, make sure to choose the route via Kandersteg and the old tunnel. The Basistunnel is much faster.

If you want a scenic route, my suggestion would be to use your BO Pass to get to Luzern, and then take the Gotthard Panorama Express from there to Bellinzona:

https://www.sbb.ch/en/leisure-holidays/travel-in-switzerland/panorama-trips/travelling-gotthard-panorama-express.html

This is a combined boat plus train ride on the old Gotthard Pass route, which utilizes some short tunnels ( some of them spirals) to cross the Alps, but still affords lovely views of peaks and hanging glaciers. Nothing like the new Gotthard Basistunnel which whisks you under most of the mountIns to shorten the time. It ends at Bellinzona, s you catch another train from there to Lugano.

I suspect this would add a considerable amount of time ver the fast 5-hour route but it should be lovely, especially with the boat trip from Luzern to Flüelen.

Thanks so much! You've been very helpful!