Courtesy Metropolitan Transportation Authority/Patrick Cashin
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As people flock to urban centers, engineers will need to figure out how to accommodate them. In 2006, New York’s Metropolitan Transportation Authority started digging a train tunnel beneath Manhattan to connect the Long Island Rail Road, the nation’s second busiest commuter train system, to Grand Central Terminal. Where the two systems will meet, at a new station 14 stories below ground, two 85-foot-tall caverns are currently under construction.

This article originally appeared in the February 2014 issue of Popular Science.

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