We have the Swiss Rail Pass. I have the adult pass and my 24 year old son has the youth pass. Is the train to Mont Blanc covered by the pass? If we stay overnight, is the return trip covered as well?
Do you mean the Swiss Travel Pass? If it is the Swiss Travel Pass it should have come with a map of where it is valid
It would be valid up to the last border stop before you get to France
This is the area of validity with the Swiss Travel Pass
https://cdn-doc.myswissalps.com/docs/default-source/rail-network-maps/swisstravelsystem.pdf?sfvrsn=7788603a_36
The Swiss Travel Pass is valid between Chamonix and Montreux, and reverse. We used it last year in June with no problem.