raising the terror alert for trains and buses in the US is a good thing, but the chances are that whomever set off the bombs in London aren't going to do it here, at least not now. if they had intended to set off bombs on subways and buses here, they would have done it in conjunction with the bombs in London. they go for spectacular, not conventional. setting off bombs in the US on trains or buses now would be conventional.
note that terror-ism is specifically that, terrifying. anticipated attacks are not terrifiying as they are predicted; unexpected, unanticipated attacks are terrifying because they are unexpected and unanticipated.
i think we can rest assured that there will be no bombs on trains or buses in the US attacked (at least not anytime soon); having bombs go off in London and somewhere in the US on the same day, within enough time of each other for someone to notice that they were related, would be the kind of spectacular terror that they go in for. that didn't happen, so it ain't gonna.
note also that they do these kinds of things (9-11, Madrid, now London) to demonstrate that the greatest powers in the world cannot stop them. once the populace figures this out, their attacks will be even more terrifying because it could mean anywhere, anytime. they have demonstrated that ability and they will continue to do so.
i'm not sure that the powers-that-be have figured this one out, yet. could be that their hubris prevents them from seeing this truth.