This article discusses attacks on railroads by al-Qaeda, "possibly using operatives who have a Western appearance," to try "to destroy key rail bridges, derail trains or target hazardous-material containers."

The most obvious method of attack by a suicidal terrorist would be parking a large truck, either something massive like a load of I-beams or something flammable like a load of gasoline, next to any one of the tens of thousands of rural railroad crossing gates across the nation. When a train comes--usually at full speed in rural locations--wait until it's close enough, and pull the truck into its path, right through the flimsy wooden gates, if any. A big train CANNOT stop in less than about a mile from top speed. It may even be possible for the terrorist to get out in time if done quickly, but this is obviously not important to the terrorists.

Boom.

There is NO WAY to defend against this, short of near-omniscience. The country is covered with rail crossings, most of them dirt-road rural, and many without active gates. A truck parked next to a crossing for more than a few minutes without a train coming would be suspicious, and an engineer who saw one a great distance away and refusing to cross, might be suspicious enough to start slowing down. But sufficient knowledge of train schedules, or a spotter with a cell phone, and the truck's arrival at the crossing could be timed precisely to avoid having to wait a long time at a gate. In any case, no one else in the area would probably notice a truck parked at a rural crossing for many minutes, unless you were in a car right behind it.

As for the trucks, anyone can get a job driving them, or one could easily be rented or even purchased outright if enough money is available. If the terrorists own their own truck, a tanker could be filled with something even deadlier like a toxic substance or a fertilizer explosive, although if they're clever they would not do this because the gathering of the material would in itself attract suspicion.

Best targets: cross country passenger trains. Big ones. Freight trains are more common, but would result in largely property damage and few deaths.

Such an attack would be extremely easy to coordinate at multiple sites at the same time, similar to the way the 9/11 hijackings were coordinated. The beauty of it, if one can warp such a term sufficiently, is that no alerting crimes are required ahead of time, like the hijackings. A few dozen unmarked trucks, moving down many roads. Who would know that all of them were converging on crossings at once?