From the official alert:

"U.S. mass transit and passenger rail systems are vulnerable to terrorist attacks because they are accessible to large numbers of the public and are notoriously difficult to secure."  Previous rail attacks in Madrid, London and Mumbai "could inspire terrorists to conduct similar attacks in the United States," the report adds.

Sounds pretty scary, right?  Madrid?  London?  Mumbai?  Bad times, those.  They certainly "could inspire" a terrorist attack.  Personally, another season of "House" may do the trick for me.  But wait, there's more:

However, the authors of the intelligence analysis make clear that there are no known, immediate dangers. "At this time, there is no credible intelligence regarding specific plans by any extremist groups or individuals to perpetrate an act of terrorism against the U.S. mass transit system," they write.

Very helpful, these alerts.

Think national security won't be an issue in this election?

Does anyone remember the last time we had such an alert?  I honestly don't remember.  I'm guessing probably the autumn of 2006 though.