page_title: Technology of the New York City Subway
section_title: Construction methods
hierarchical_section_title: Technology of the New York City Subway / Construction methods
caption_attribution_description: Workers are building the extension of the 7 subway line to 34th Street and Eleventh Avenue. This photo shows the progress as of June 14, 2011: A completed tunnel with its concrete liner in place. Photo by Metropolitan Transportation Authority / Patrick Cashin.
context_page_description: Since the late 20th century, the Metropolitan Transportation Authority has started several projects to maintain and improve the New York City Subway. Some of these projects, such as subway line automation, proposed platform screen doors, the FASTRACK maintenance program, and infrastructural improvements proposed in 2015–2019 Capital Program, contribute toward improving the system's efficiency. Others, such as train-arrival "countdown clocks", "Help Point" station intercoms, "On the Go! Travel Station" passenger kiosks, wireless and cellular network connections in stations, MetroCard fare payment alternatives, and digital ads, are meant to benefit individual passengers. Yet others, including the various methods of subway construction, do not directly impact the passenger interface, but are used to make subway operations efficient.
  In the mid-1990s, it started converting the BMT Canarsie Line to use communications-based train control, using a moving block signal system that allowed more trains to use the tracks and thus increasing passenger capacity. After the Canarsie Line tests were successful, the MTA expanded the automation program in the 2000s and 2010s to include other lines.
context_section_description: When the IRT subway debuted in 1904, the typical tunnel construction method was cut-and-cover. The street was torn up to dig the tunnel below before being rebuilt from above. Traffic on the street above would be interrupted due to the digging up of the street. Temporary steel and wooden bridges carried surface traffic above the construction. The 7,700 workers who built the original subway lines were mostly immigrants living in Manhattan.
  Contractors in this type of construction faced many obstacles, both natural and man-made. They had to deal with rock formations, and ground water, which required pumps. Twelve miles of sewers, as well as water and gas mains, electric conduits, and New York City steam system pipes had to be rerouted. Street railways had to be torn up to allow the work.  The foundations of tall buildings often ran near the subway construction, and in some cases needed underpinning to ensure stability.
  This method worked well for digging soft dirt and gravel near the street surface. However, tunnelling shields were required for deeper sections, such as the Harlem and East River tunnels, which used cast-iron tubes. Segments between 33rd and 42nd streets under Park Avenue, 116th Street and 120th Street under Broadway, and 145th Street and Dyckman Street (Fort George) under Broadway and Saint Nicholas Avenue as well as the tunnel from 96th Street to Central Park North–110th Street & Lenox Avenue, used either rock or concrete-lined tunnels.
  About 40% of the subway system runs on surface or elevated tracks, including steel or cast iron elevated structures, concrete viaducts, embankments, open cuts and surface routes. All of these construction methods are completely grade-separated from road and pedestrian crossings, and most crossings of two subway tracks are grade-separated with flying junctions. The sole level junctions of two lines in regular revenue service are the 142nd Street junction and the Myrtle Avenue junction.
  More recent projects use tunnel boring machines, which minimize disruption at street level and avoid already existing utilities, but increase cost. Examples of such projects include the extension of the IRT Flushing Line and the IND Second Avenue Line.
caption: Recent projects, like the extension of the IRT Flushing Line (pictured) use tunnel boring machines to build the subway tunnels.
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