In New York City today 1.5 million people daily jam the closest existing subway line on the East Side - that's more daily riders than are carried by the metro rail lines of Washington, Boston and Chicago combined.

The Second Avenue Subway was first proposed in the 1920s, when the cost of a line running from Houston Street to the Harlem River was estimated at $86 million.

Left: 1925 views of the elevated tracks of the Second Avenue line in Manhattan.

The Second Avenue El was discontinued in 1942 and the overhead tracks demolished; the Third Avenue elevated lines were taken down in the Fifties. But work on the Second Avenue Subway line - delayed first by the Great Depression - was continually put off because of wars and economic woes that sidetracked the most important thing needed: money. After a groundbreaking ceremony in the early '70s, construction was halted due to the city's financial problems.

Phase 1 of construction, begun in 2007, included the excavation of new tunnels eight stories beneath Manhattan's streets, from 92nd to 63rd Streets, as well as access shafts at 69th and 72nd Streets. Service to the West Side and Midtown will also be provided via connection tunnels underneath Central Park (not currently used for passenger travel).

By the time Phases 3 and 4 are completed, the new T line will run from 125th Street to Hanover Square.

Three different excavation methods are being used. Much of the subway is being built using tunnel boring technology - powerful circular cutting machines that drill through bedrock or soil.

The excavated material is then lifted to street level for removal by truck.

The Tunnel Boring Machine (TBM).

The Tunnel Boring Machine is assembled.

The 485-ton, 450-foot-long Tunnel Boring Machine is assembled.

The Tunnel Boring Machine used a 22-foot diameter cutterhead to mine 7,789 linear feet in two tunnels.

The TBM averaged approximately 60 linear feet a day.

A construction worker is seen at the Launch Box during East Tunnel mining operations.

In September 2011 workers completed tunneling the first phase of the Second Avenue Subway when the tunnel boring machine reached the Lexington Ave.-63rd St. Station, connecting to the existing subway system.

Workers are seen as the TBM broke into the existing subway system at the completion of the first phase of the Second Avenue Subway, Sept. 22, 2011.

This arch form is used for tunnel concrete operations.

Waterproofing of the east tunnel of the Second Avenue Subway.

The "Horseshoe" tunnel at the 72nd Street station site looking south.

Carving out an entrance to the cavernous space that will become the Second Avenue Subway's 72nd Street Station.

During construction, air quality monitoring stations have been installed to measure the continued effects of construction.

The 69th Street South Crossover, looking south.

The "cut-and-cover" method is being used for much of the line, where a trench is cut in the street, the soil supported by vertical walls. This framework allows the street to remain open to traffic and pedestrians while excavation and construction continue below.

A view of a trench at 72nd Street.

A bird's eye view of work on the Second Avenue Subway's 96th Street entrance, with decking installation underway, January 24, 2012.

The new tunnel required the removal and replacement of a water main.

Sewer and gas mains also had to be relocated.

Excavation at the site of the 69th Street Station.

Watering during drilling operations at 86th Street.

For the mining method, the primary process involves drilling small holes within a rock area and then using explosives to dislodge the rock. Otherwise, soil and rock will be removed by backhoes, bulldozers, or a clamshell shovel.

A bulldozer used to remove rock and soil, eighty feet underneath Second Avenue.

An underground view of tunnels at the site of the 69th Street Station.

The cavern at 69th Street and Second Avenue.

A work train is used to haul debris out of the 63rd Street dig along existing subway tracks.

Preparing new steel columns and rebar for concrete, to connect lower to upper platform levels.

The new T line tunnels will be joined to the existing B and D line service, with a common mezzanine, at a reconfigured Grand Street Station.

A firefighter stand outside a shattered window at the Kolb Art Gallery on East 72nd St. in New York, after blasting at the construction site for the Second Avenue subway shattered windows and sent smoke billowing up to the street, Tuesday, Aug. 21, 2012.

CBS Station WCBS reports that steel plates covering the Second Avenue subway construction site failed to withstand the impact of a controlled blast that sent rocks flying into the street, damaging nearby buildings. Construction workers were blasting through rock to create an escalator for the subway when two 1,800-pound steel plates were lifted into the air, allowing debris to rain onto the street.

The cover was supposed to absorb the pressure but didn't, said Michael Horodniceanu, president of capital construction for the Metropolitan Transportation Authority.

The mud-filled cavern that will house the Second Avenue Subway's 96th Street station.

On March 19, 2013, a construction worker became stuck in the mud, and was rescued by fellow workers and N.Y.C. firefighters.

Work continued in late January 2013 at the future 72nd Street Station, where crews are filling out the raw rock of the cavern with concrete lining.

For more info:
MTA Capital Construction: Second Avenue Subway

By CBSNews.com senior editor David Morgan