Metro North is making some track improvements to the Danbury branch.  But that will mean some substitute bussing for off-peak weekday trains, beginning today.  The cyclical track replacement program will continue through mid-November.  Substitute buses between Danbury and South Norwalk will operate as much as 20 minutes earlier than scheduled trains.   Bus service for the 6:48 PM train from Danbury will connect with the 8:30 PM train leaving Stamford.