So I used to be a very active graffiti writer back in the late 90's. My primary targets were local trains in general and subway trains in particular. (yeah I k

So I used to be a very active graffiti writer back in the late 90's. My primary targets were local trains in general and subway trains in particular. (yeah I know that's bad but that's not the point) Speaking from first hand experience, there's always a flaw in your security system: cameras that no one watches, broken sensors that no one fixes, alarms that no one reacts to, master keys that are easy to find and duplicate, padlocks that are easy to break and replace with our owns, sensors that are easy to circumvent, employees that are easy to bribe, witnesses that won't do a thing, a network too large and old, holes in the fence that are never repaired, trains that are never checked before going into trafic, etc. My friends and I could spend hours in a yard, often bringing ladders, doing our stuff without anyone batting a lash. And the next day. And the next one. It was only paint, and we were only a bunch of apolitical "kids" having fun, though, so no biggie. Basically, we can't be 100% safe. Patrolling the city centres, checking bags at the mall, x-rays and dogs at the airport… none of it makes anything safe, really. What makes us safe (and we are) is the terrorists' limited resources and lack of imagination.