A small comment that will probably go mostly unnoticed, but this question has always bothered me too and it's a second opportunity inside of an hour to talk about working at Disneyland post 9/11. We instituted bag checks at the intake turnstiles for the first time after that, and the result was enormous lines that backed up all the way to the streets. It left me wondering why nobody just detonated a bomb or fired the entire magazine of an Uzi into the crowd waiting to get into the park. You could still kill anywhere from hundreds to thousands of people, it's still at Disneyland. What were we really accomplishing by making it less likely to happen in the park, but possibly making it even easier to happen right outside, given the gridlock, how tightly everyone was packed, and the impossibility of hiding anywhere or running away?

That's because you can't really drive trains into buildings, and while you could technically do it with a bus, it's not that feasible nor that damaging. Flying a huge intercontinental airliner fully loaded with fuel into a building is quite different.

There's very good practical reasons the terrorists chose airplanes and not trains or buses.

It doesn't happen now, after 9/11. 9/11 was a one-time thing, and basically ruined airliner hijacking forever for other would-be hijackers. Before 9/11, people were taught to comply with hijackers because it was just a political stunt and people generally didn't get hurt. After 9/11, passengers will now fight for their lives because they'll assume the hijackers are religious kooks who intend to murder them all by flying them into a building. A few hijackers with razor blades today will be torn limb from limb by the other passengers.