alt Hacker NewsIf you're comparing HSR to air travel with post 9/11 security then there's a a moderate number. However, once you have HSR that moves a significant amount of people, you WILL get post 9/11 security there as well, taking the number to approximately zero.
Plenty of places operate much more dense transit without that level of security.
Trains, even HSR have essentially zero hijacking risk relative to airplanes, and proper safety interlocks make them very difficult to use the vehicle as a weapon.
At that point, your terrorism risk is approximately on level with any other dense transportation interconnect.