The MBTA announced today the Ashmont branch of the Red Line will be closed on Saturday so workers can remove one of those slow zones - and to make sure there aren't any potential problems left over from the Peabody Square lake that formed last week after a National Grid crew cracked open a BWSC water main there. Trains will be replaced by shuttle buses - like for the previously announced shutdown of the Braintree branch - which will remain closed on Sunday.