Sep 17, 2017 - 9:00 PM
in reply to Agrawal
You can either go round in a circle as described in previous posts or go Luzern - Chur - Tirano. The circle route will take 12 - 13hrs. The following the BEX route there and back will take an hour or more longer.
You can find circle options on SBB timetable by entering from Luzern to Luzern via Chur and via Tirano. For the other route add via Chur as well to the end of the search parameters.
I still think it would make much more sense proceeding to Venice from Tirano rather than returning to Luzern.