Congestion on the 4/5/6 feeding the Upper East Side has been a problem for as long as I can remember. First proposed in 1929, the Second Ave Subway (SAS) was designed to supply another major subway line to this always popular neighborhood.  However, the project has been a joke for a long time:  always discussed, never built.    Finally in 2007, this first major addition to the NYC subway since the 1940s began.  The project started and stopped several times over the years, so the current design includes digging new tunnels as well as reclaiming old tunnels and sections of track that were partially built.  Information about construction and the progress is available on the MTA’s website for the SAS.  There’s also a decent wikipedia entry on the subject that gives the sorted history of the project that almost was for 80 years.