R. and I kicked around whether there was a parallel problem with asphalt roads. He commented on bridges, and I said bridges are bridges -- how about the _road_. He mentioned a couple of methods for damaging the road, but none were particularly compelling. And then he recalled the rocks-off-the-highway-overpass problem.
Yeah, that's about right. It's damn easy to do. It kills people quite successfully (people shooting at cars, similarly, whether shooting from in another car or by the side of the road), whether the person meant to or not. And it's fucking expensive to prevent.
So anyone suggesting that it would be too dangerous to run fast passenger trains because of sabotage risk and/or too expensive to put in place measures to prevent sabotage (grade separation, fencing, automated surveillance, etc.) is going to have to run a comparison against people killing motorists by doing things like dropping rocks off of overpasses.