The common argument against overnight service in non-New York cities is "only two tracks! no time to do maintenance!": http://www.bart.gov/guide/latenight
This seems to just completely ignore the existence of the 14th Street-Canarsie and Crosstown lines (L and G services), both of which are dual tracked, yet provide 24/7 service most of the time. The Second Avenue Subway will also be dual tracked (unfortunately).