There's nothing wrong with mentioning brand names or the like here; it's merely spammers who get the axe.

In the USA, one can purchase the Pacsafe cable lock (uses a found number combination, easy for the owner to unlock, not so easy for others to try the "spin of fortune", and the cable coils inside the lock for space-saving) for under $20 at some travel stores, REI, etc.


I have used this on occasion in 2nd class travelling through some dodgy areas; it doesn't stick out visually (in black, which it is available in,) and of course a small lock on the zipper helps as well. It's merely a deterrent, of course; nobody prepared and committed will be stopped (but it will take a little time so they will feel inhibited and visible).

One thing that makes it easier to keep an eye on baggage in a train or even in an airplane - put the bag on the rack across the aisle from you.