Rivaling the U.S.S. Intrepid in length, but invisible to passers-by or office workers looking out their windows, parallel twin tubes now stretch from 10th to 11th Avenue, just west of Pennsylvania Station. Two stories tall, each of the tubes is large enough to accommodate an Amtrak or New Jersey Transit train, which is what they are intended to do some day in the future.

There has been a lot of talk lately about the multibillion-dollar Gateway project, which would double the rail capacity into and out of Penn Station by adding a two-track tunnel under the Hudson River, and two bridges over the Hackensack River in New Jersey.

But Gateway is more than just talk, as you learn after descending an extension ladder through a hatchway barely big enough to contain you, your safety harness and the ductwork needed to bring fresh air underground.

“Welcome to our cavern,” James McCarron said, his voice resonating down one of the 850-foot-long tubes in a concrete casing underneath the Long Island Rail Road yards on the Far West Side of Manhattan, deep below the platform from which the vast Hudson Yards development is rising.