They do. They do some work on nights and weekends, and some lines have shut down and will shut down for periods of months. The L train (Manhattan to Brooklyn)

They do. They do some work on nights and weekends, and some lines have shut down and will shut down for periods of months. The L train (Manhattan to Brooklyn) is scheduled to shut down for over a year to do residual repairs from Hurricane Sandy's flooding. But it's a major pain. The system is at capacity many hours of the day, as are surface roads, and some points in the city are only near one line, so there's no easy way to get people where they need to go when their train can't run.