The New York Times says that many of New York subways are running at full capacity. A chart indicates that, on the 2, 3, 4, 5, and E lines, the trains are full and they can’t run any more trains per hour.
The Antiplanner has written about transit capacities before and pointed out that an exclusive bus lane running at freeway speeds can actually move more passenger miles per hour than a subway. However, no one expects New York City to build bus lanes running at freeway speeds on Manhattan.