High Speed Rail would have the EXACT same security measures as airplanes, except they would be even less safe as blowing up track is easy, especially when you have hundreds of miles to choose from.
How do you explain that, in the countries that have high speed rail, the airport security is the same as in the US but there's no such thing for trains?
Bombing a plane is already not the most effective way to kill people, it's mostly for show. Attacking a subway would still work better than attacking a high speed rail (or low speed rail for that matter, I don't see why that would make a difference except for popularity, I guess)