I'm not aware of any underground (or above ground) system that has a 30 second headway unless you're talking about a gondola which is more close to the capacity that this system can handle.
Yes, trains can hold more, but that's not the point of this system. If you build a 1500 person train, you now have to build the stations and platforms to accommodate those kinds of crowds.
Edit: apparently in the Crossrail it will be every 2.5 minutes, but I remember that in Moscow it is every minute.