what's up with raising the terror alert for just NYC subways???

I just heard on the radio that the government has information gathered in Iraq, abour credible threats on the subways. They are apparently date, time & location specific, but the police are giving those details. It is confirmed that it is a bomb threat.
They have put more police in the subway stations & are checking bags more than usual.

I hope this turns out to be a false alarm...that would be devastating