The thing is in parts of New York City 5 min gaps aren't possible in the rail lines even at peak. The reason is traffic in a manner of speaking. The New York C

The thing is in parts of New York City 5 min gaps aren't possible in the rail lines even at peak. The reason is traffic in a manner of speaking. The New York City subway uses extensive interlining which means that two or more services share track for part of their route (typically the part in Manhattan). If trains on the shared track are every 3 min, by conservation of mass, trains on the split part need to be every 6 min. NYC also is still upgrading it's signaling system so for now train are a 5 min interval even in the best parts and 10 min intervals everywhere else. There is physically no easy fix.