Comment Cable Lock (Score 1) 293
It depends on how strong you want your security to be. While traveling by train in Europe, I generally use a simple cable lock (like the Master Lock 4605D, $10 at Amazon). It's small and easy to deploy. Obviously, a determined and well-equipped thief could cut through the cable pretty easily, but it's enough to deter the casual grab-and-dash artist. I've used it for twenty years, and I have yet to lose a bag. Of course, having said that, my next trip will be the one where I encounter a well-equipped thief...