sure, they're all subject to terrorism. but none of the modes you mentioned can result in the death of 5,000 people if only a few people have a sharp object in their possession.
at least the other things you mentioned require more involvement that can be expensive, require more expertise, or greater logistics.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by NoStressHere:
Here is a a thought.
Do we need screeners at all?
You can get on a train without screening.
You can get on a bus without screening.
You can drive on a very busy freeway without screening.
You can drive up to the terminal without screening.
You can walk into an airport terminal without screening.
You can get on the subway without screening.
You can send mail without screening.
Think about it.</font>