Best contact the train company that drove you there immediatly. This will probably be SBB - CFF - FFS. It should be the IR90, with end station Brig?

Usually lost stuff ends up at the final station of that specific train, where you can collect it if you can prove it’s yours. Call SBB asap, or find the nearest ticket office to explain your situation. Usually collecting lost stuff comes at a small cost.

Did you forget the backpack or was it stolen?

If it was stolen you must make a police report and then check how to get a temporary new passport. 

If you forgot it you should go to the nearest SBB station and ask if they can call the train and see if the staff can locate your backpack.

Here is a link to SBB lost and found.

https://lostandfound.sbb.ch/home/sbb/SBB/app?Culturename=en

You can also use this link: https://www.sbb.ch/en/help-and-contact/lost-found-office/submit-loss-report.html

Of course it could also be forgotten to take off train and got stolen by some other pax who left later.

Start first with contacting SBB, if they say nothing has been reported, then file police report-in theory this should be done in the country where it happened. IF this is the case, do not have high hopes on ever seeing it again.

Thank you all for the help. I went to the end of the line of that train and checked the lost and found at Brig but nothing had been turned in. I’m starting to think maybe I left it at the train station at montreux, I am trying to find if they have a station number to check CCTV. 

Here is the phone number to the ticket office in Montreux. They should be open until 19.00

Here is the phone number to the ticket office in Montreux. They should be open until 19.00

 

This is just the normal number with the ripoff fee and endless waiting time. better ask at the next station to call Montreux. they have direct numbers (which they dont handout)