In fact, single-tracking in most of the subway degrades service even more than it technically needs to, due to the inability of the interlocking system to rever

In fact, single-tracking in most of the subway degrades service even more than it technically needs to, due to the inability of the interlocking system to reverse track directions. To single track the MTA has to tie down the stop arms and coordinate by radio, and it is almost never worth it. The MTA does sometimes use single tracking in the East River tunnels, where the interlocking allows the track direction to be reversed. For comparison, the entire PATH system supports changing the track direction, and single-tracking is relatively common during weekends. Of course, the PATH is much smaller and has a different history behind it, not to mention totally different legal requirements (it is a "railroad" rather than a heavy rail system).