It’s an active construction site, hard hat and harness required, but the two tubes under the rising towers of the sprawling Hudson Yards development on the Far West Side of Manhattan are relatively peaceful. The nine-foot thick concrete walls, floors and ceilings are all smoothed over, humming with the whir of ventilation ducts and the low buzz of battery-powered LED lighting.

Echoes from shouts reverberate for seconds.

For two blocks, the box tunnels stretch, curving slightly as they cut a clean concrete cavern 90 feet below 10th and 11th Avenues, plugged on both ends by stark concrete walls that abruptly halt their march.

For now, the tubes sit empty and unused. But someday, they might be key pieces to providing the relief that many rail travelers have long yearned for — the tubes would help link a new tunnel under the Hudson River to tracks leading to Pennsylvania Station.

“What you see here is the concrete casing, complete here in the first two sections,” Craig Schulz, a spokesman for Amtrak, explained to Representative Josh Gottheimer, a Democrat from New Jersey, as he toured the project on a recent weekday. Construction has yet to begin on one remaining section that would connect the box tunnels to the Hudson tunnel.