I'm oversimplifying a tad -- in reality, a piece of steel heavy enough to derail a train would probably be too heavy for me to move without a hand truck -- but if I really were of a murderous mindset, I doubt I'd need long to think of something. I doubt anybody would, which is why I'm unimpressed by reports that Bin Laden's computer held word of a possible train-derailment plot on the tenth anniversary of 9/11.
There's over 140,000 miles of standard-gauge train track in the USA (according to a quick Wiki search) and no way in hell anybody can watch it all. But we don't need to; for as long as railroads have existed, anyone with access to a big stone or metal bar, and the will to cause harm, could derail a train anytime they wished. And al-Qaeda's been reduced to train-derailment plots; i.e., the sort of stunts I personally could pull right now if I wanted to derail a train in lieu of finishing this post and going to bed. It is good that they finally got Bin Laden, but in the same way it's good when Germany still arrests the occasional incontinent ninety-something-year-old ex-concentration camp guard: yes, he's responsible for great evil and deserves to face consequences; yes, it's a travesty of justice that he remained free as long as he did; but no, we didn't make the world safer by taking him out of it.
The terror threat is the same as it's been since the aftermath of 9/11: negligible. The 9/11 attacks happened because terrorists exploited a loophole that will never be opened again, at least not while anyone who personally remembers that day is still alive. Ever since then, the only thing we've had to fear is fear itself -- and the nasty things fear is leading America into.