I was trying to explain to my mother why security on subways can not be handled the same way as at airports. First of all–my God. That would be terrible. And second, look at this picture of the Moscow system:
Now that is geographic coverage. And with that many stations and 7 million passengers a day, we’re fortunate that the attacks–which were horrible enough as it was–weren’t far worse: CNN has some commentary: Why no subway is safe from terror attacks – CNN.com