>It's interesting that the same security measures are not even considered for mass rail transit. Set off a device in the right location underground and it could be equally damaging...

underground is mostly for people trains - only ABC will get you high body counts.
for kinetic aka chemical bomb the needed mass for property destruction is to great.
There is a reason for car bombs... Airplanes by themselves already have a
nice kinetic vector and some ingitable material already packaged in
and are a bit more manuverable as well ;-)

Loaded high speed trains might be an edge case for the people inside...

regards

thomas