Sure they could kill everyone on the plane, but a train has more passengers than a plane, and a determined terrorist could probably kill everyone on a train.
They don't really need to hijack planes, they just need a few hundred thousand dollars and a NetJets account. Also, if you read some of the BlueLeaks documents you can tell the government is very worried about vehicle ramming attacks. The unfortunate truth is that anyone with a little bit of money and willingness to die for a cause if coordinated can cause massive amounts of damage. We are lucky that most terrorists aren't that smart.