For the Northgate Link extension project, the paired tunnels constructed for University Link will continue north to the University District and the Roosevelt neighborhoods before emerging at the surface along I-5 near NE 94th Street to serve the elevated Northgate station. Sound Transit is preparing to extend mass transit from Northgate to Lynnwood, which voters approved as part of the Sound Transit 2 Plan in 2008, along with funding to continue planning future service all the way to Everett. This project is known as the Lynnwood Link extension project.
Stations
Construction
Construction on the Northgate Link extension began in 2012 and is scheduled to open for service in 2021.
Tunnels
Each tunnel is dug southbound, starting from the Maple Leaf Portal, in three segments separated by stations, using a diameter tunnel boring machine. The machine digging the tunnel for northbound trains was named "Brenda" and the sister machine for the southbound trains was named "Pamela", but the names were dropped in March 2016 to reduce associations with the troubled "Bertha" machine used on the Alaskan Way Viaduct replacement tunnel project. Brenda was previously used on the University Link Tunnel from 2011 to 2012, digging both tunnels between Capitol Hill station and the Downtown Seattle Transit Tunnel. TBM #1 began excavation of the northbound tunnel on July 9, 2014, and reached Roosevelt station on March 17, 2015. It later reached the U District station on November 6, 2015, and completed the entire northbound tunnel to University of Washington station on March 30, 2016, less than two weeks after light rail service to that station from Downtown Seattle had begun. TBM #1 was disassembled and removed from the University of Washington station. Due to damage sustained by TBM #2, TBM #1 was refurbished and excavated the last tunnel segment between June and September 2016. TBM #2 began excavation of the southbound tunnel on November 20, 2014, and reached Roosevelt station on July 13, 2015. For six weeks from late December 2015 to early February 2016, the machine was stopped at a point north of U District station after encountering hard soil that damaged five motors and the main gear of the machine. Repairs were made and the segment was completed on March 24, 2016. Additional repairs to TBM #2 needed to complete the final tunnel segment would have potentially delayed the project's opening if the five months of float time were exceeded, leading JCM Northlink to finish the final segment with TBM #1.
