Recently my backpack fell off a luggage rack and straight across my shoulders. It was a 30 pound reminder of a principle I had forgotten to employ: Always strap up your backpack on the racks. It will keep you protected from such an incident and foil any opportunistic thief who attempts a quick grab and run. The longer the crook has to fumble with your clips, the higher the chance they will abort mission. Face all zippers away from the aisle.
Additionally keep your wallet/phone/passport away from the pant's pocket which faces the aisle. Zipping them up in an internal jacket space is the ideal.
Now 99% of the folks you meet in public transpo are friendly. In consideration of these good guys grab a bag of snacks at a kiosk before your journey. It's amazing how much good will a few pistachios or a tiny candy can generate. The appreciative locals are now likely to share something with you, wake you up when your stop arrives, and scare off the big bad thief after he discovers your bag has been strapped to the rack.