Considering I’m in the heart of Chicago and 1 million plus people are headed down here for fireworks this evening - I’m really hoping to get out of here early today!
They indicated that they chose the cities generally based on population densities (and probably passenger volumes as well) and not on any unique information on threats pertaining to these particular locations. If they had “specific and credible” information that any of these cities were a target, they would (at the very least) raise the alert level for specific types of mass transit (as they have with airports for the past couple years) or specific cities (the 2004 NYC/DC/Newark threats). If the threat were specific and credible enough, they could raise the general threat level in certain locations or even nationwide to “orange”, and in the worst case, “red” (but they really don’t want to do either unless absolutely necessary because it causes “alert fatigue” aka “the boy that cried wolf”)
If they come across new information from this ongoing situation with these international networks being uncovered, I expect a new alert to come out. But unless they come across that type of specific and credible information, the threat is probably no greater in these mass transit systems and airports than it was 10 days ago or will still be 10 days from now (not that such a possibility is necessarily a comforting thought). The problem is that vaguely worded “September 10th threats” like “the match is tomorrow” with no way of knowing the source of the threat may make sense to everyone after the fact, but before the fact, it does not help much. Hence the need for “specific and credible” information.
Stay safe.