Actually, in much of the system there is a great deal of redundancy so one track can be shut down for maintenance using weekends and 10 PM to 5 AM. For example there are often parallel local and express tracks (6 local, 4,5, express or 1 local and 2, 3 express or C local and A express).

Also entire train lines frequently run in parallel a few blocks from each other.

See: http://web.mta.info/maps/submap.html