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.

That is already completely mitigated by the fact that cockpit is sealed now days, so hijackers cannot kill/disable the pilots and take over.

Rail related terrorism is not uncommon.

https://en.wikipedia.org/wiki/List_of_terrorist_incidents_in...