The tunnel at the 63rd Street and Lexington Avenue station is being expanded to run trains from the planned Second Avenue Subway. Metropolitan Transportation Authority/Patrick Cashin
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New York City’s Second Avenue subway line was the greatest project the city never built — until 2007, almost 90 years after its initial proposal, when the city finally broke ground on it. When completed, the $4.45 billion project will provide New Yorkers with two more miles of commutable subway tunnels on Manhattan’s East Side.

Photos released on the MTA’s Flickr page show a few pieces of the new route in-progress–and just how hard it is to dig a miles-long tunnel under a densely-populated island city.

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The tunnel at the 63rd Street and Lexington Avenue station is being expanded to run trains from the planned Second Avenue Subway.

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At the future 72nd Street station, workers are lining the raw rock of the cavern with concrete.

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Part of what will be the 72nd Street Station.

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Construction on Phase 1 of the project is due to be completed in December 2016.

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When completed, the new line will consist of 2 miles of new tunnels.

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A Second Avenue subway plan has been on the books since the 1920s.

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Before construction started in 2007, it was sometimes called “the line that time forgot.”

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The project is expected to cost $4.45 billion, according to the MTA.