Despite worry over bad boring reputations, one Seattle machine is celebrating yet another milestone after breaking through a wall in the Univeristy District.

The tunnel boring machine is mining the tunnel for northbound light-rail trains to Northgate. It is one of its two separate 3.5-mile tunnels.

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Sound Transit says the 600-ton machine holed through the wall of its retrieval shaft at the University of Washington light rail station as part of the Northgate Link Extension.

“Our tunnel projects are on track to connect more people to fast, reliable mass transit,” said Sound Transit Chair and King County Executive Dow Constantine. “This latest breakthrough at Husky Stadium brings us another step closer to extending high-capacity light rail to Northgate by 2021.”

This was the fifth of six tunnel segments being mined by JCM Northlink LLC, Sound Transit’s tunneling contractor for the extension project. The TBM No. 1 machine had launched from the Maple Leaf Portal at NE 94th Street south of Northgate Mall in July 2014. Another tunnel boring machine, TBM No. 2, launched from the Maple Leaf Portal in November 2014 and reached the U-District Station site last week.

Both previously bored tunnels will now undergo a thorough inspection. Once inspections are complete, the contractor will determine which machine will be used to mine the final tunnel segment to the next Univeristy of Washington station at Husky Stadium. Crews will then spend 2-3 months refurbishing the selected TBM to prepare for its final launch.

Tunneling is expected to be complete by the end of this year. When light rail service on the Northgate Link extension begins in 2021, trains will enter and exit the tunnels in the Maple Leaf Portal.

The $1.9 billion extension project will connect the University of Washington Station at Husky Stadium to underground stations in the U-District and Roosevelt neighborhood, and on to an elevated station at Northgate. It is expected to provide 14-minute rides to downtown, seven-minute rides to Husky Stadium, and 47-minute rides to Sea-Tac Airport.