Building new subway lines is a massive undertaking and this gallery of photos gives some indication of the scale of the work. Here is what New York is undertaking:
The Metropolitan Transportation Authority (MTA) has taken on three massive projects: East Side Access, the No. 7 subway line extension, and the new Second Avenue subway line. Construction for the projects is taking place deep underground, much of it simultaneously. The three projects span 14 miles and are expected to be finished in 2019 at an estimated cost of $15 billion.
This indicates a few things: New York has the kind of capital and ability to build such lines and the trouble it takes to construct such things suggests new lines must be needed in a city where a large percentage of residents use mass transit on a daily basis.