The boring of twin 3.5-mile tunnels to carry light rail trains between Northgate Mall and the University of Washington hit a key milestone with the tunnel-boring machine Brenda breaking through the wall of Sound Transit’s future U District station.

Brenda completed the third of six tunnel segments in the JCM Northlink LLC—a joint venture of Jay Dee Contractors, Coluccio and Michels—contract to extend light rail to the university. Brenda launched in July 2014 from the Maple Leaf Portal near Northgate Mall. The breakthrough is at the spot of the future U District station.

With Brenda coming up for air, crews can now put the machine through maintenance before sending it along in its southward pace to the planned University of Washington Station.

A second machine named Pamela launched from Maple Leaf Portal in November 2014 and should arrive at the U District site in early 2016. All tunneling should wrap by the third quarter of 2016, Sound Transit says. Then, with tunnel finishes completed in the first quarter of 2018, Sound Transit expects to carry passengers on the extension in 2021.

To date, Sound Transit has bored nine tunnel segments for Link light rail construction in the region. Mining of the twin tunnels serving the Beacon Hill Station were completed in 2008. Mining of parallel tunnels between downtown and the University Link Capitol Hill Station and the Capitol Hill Station and University of Washington Station finished in 2012.

When the remaining three tunnels for Northgate Link wrap up, Sound Transit will have mined a total of 12 tunnels for light rail.

Each of the current machines weighs 600 tons and is more than 300 feet long, including the trailing gear. The 21.5-ft-dia cutterheads will eventually churn more than 500,000 cu yards of soil en route to installing 7,200 concrete rings.

The $1.9 billion Northgate Link Extension will connect the university at Husky Stadium to underground stations in the U District and Roosevelt neighborhood to an elevated station at Northgate. The 4.3-mile extensions will provide a 14-minute ride to downtown, seven-minute ride to Husky Stadium and a 47-minute ride to Sea-Tax Airport.

Follow Tim Newcomb on Twitter at @tdnewcomb