NORTH BERGEN, N.J. — The Gateway Development Commission began assembling tunnel-boring machines at a North Bergen, New Jersey, site on Tuesday for the $16 billion Hudson River rail tunnel project. The completed tunnel will run almost 2.5 miles, or 4 kilometers, with two train tracks inside, and is expected to carry trains by 2035, easing a bottleneck on the busiest passenger rail corridor in the nation, if all goes as planned.

The drilling machines stretch nearly the length of two football fields and are equipped with cutters harder than diamonds to excavate dense rock. A crew of about 40 workers will oversee a conveyor system removing debris and equipment used to install the tunnel's curved concrete lining. "This is a fully automated, underground factory," said James Starace, chief of program delivery for the Gateway Development Commission, who stood Tuesday in a trench cut into the hills across the Hudson River from New York City wearing a hardhat and yellow safety vest.

The tunnel-boring machines arrived from Germany in nearly 100 different components, said Hamed Nejad, chief engineer for the project. Outside the tunnel's future entrance, a team of welders fused pieces of the cutterheads.

Starace said the machines are expected to take about a year to grind through the first section of volcanic rock in the New Jersey Palisades once digging starts later this year. The machines will advance at about 30 feet of tunnel per day, while other machines will dig under the riverbed. The original two-track tunnel, damaged by saltwater during Superstorm Sandy, will be renovated. The original tunnel was built more than a century ago using picks and shovels from each side that eventually met in the middle.

The project received key approvals and funding under the Biden administration. The Trump administration froze funding for the project during a recent federal government shutdown, citing concerns over diversity, equity and inclusion-based fund allocation. In February, as project funding neared depletion, a federal judge ordered the Trump administration to release the funds. Funds have continued flowing to the project during an ongoing lawsuit by New York and New Jersey against the federal government.

"What's astonishing about Gateway isn't the size and scope of the project, but that it's taken this long to get only so far," said Danny Pearlstein, spokesperson for Riders Alliance.