Why not? You don't have to go through these checks when getting on BART, Muni, or Greyhound. I regularly board high speed trains here in Europe with only rand

Why not? You don't have to go through these checks when getting on BART, Muni, or Greyhound. I regularly board high speed trains here in Europe with only random security checks (usually dogs roaming the stations). Airplane security for all passengers came about primarily because of hijackings, and was later adapted (badly) to bombs and suicide tactics. Planes are a more suitable target for these types of attacks because of many reasons, including the virtually unlimited amount of possible destinations (unfortunately also including the ground or buildings). Even a small explosion on a plane can bring it down, and there is already an existing factor of fear when flying. The current level of airline security is still ridiculous, given all of this, but it would be doubly so on a train system. Hijacking a train is borderline pointless, crashing one into something is next to impossible, and derailing from on-board would be a pretty large challenge. Trains are also at ground-level, which is less than ideal for hostage scenarios. Trains are only a sexy target because they have a large number of people on them, and even then people are relatively spaced out along a long line. In my opinion this makes them even less attractive to a terrorist than any other crowded space.