Sixteen stories below Manhattan's Grand Central Terminal, an army of workers has blasted through bedrock for a new commuter rail concourse with more floor space than New Orleans' Superdome.

Future Hudson Yards station (YouTube)

It's one of three bold projects under way below New York City that will expand what's already the nation's biggest mass transit system by 2019.

The 350,000-square-foot concourse below Grand Central will accommodate Long Island Rail Road trains now bypassing the East Side on their way to Penn Station on the West Side.

On Manhattan's far East Side, the Second Avenue Subway is being built to ease rider congestion on Lexington Avenue trains.

And an extension of the No. 7 subway line will open next year from Times Square to a huge new West Side development called Hudson Yards.