A key component to a successful operation is not to be detected. National mass transit (particularly rail) is at 'ORANGE'. Furthermore, the eyes of Americans are more attentive, now that the London attack has occured. With all this attention, the chance of a similar attack in the near future is very slim. That said, I think a rail-bombing attack is now a higher potential method than I had earlier considered.
I truly hope you're right. I took NJ Transit into NYC this morning and the commuters I observed were reading their papers and sleeping as usual. Most passengers on the train carry a bag or a backpack. I just don't think this type of an attack on the transit system can be avoided, short of checking bags (which would be a nightmare).