It serves intermediate stations at U District and Roosevelt before emerging on the east side of Interstate 5 at a portal in Maple Leaf.

The extension was approved in 2008 by voters in the Sound Transit 2 (ST2) package, began construction in 2012, and entered service on October 2, 2021.

[3] An extension north to Lynnwood was approved as part of the Sound Transit 2 ballot measure in 2008, along with funding to continue planning future service all the way to Everett.

[11] TBM #1 (formerly Brenda) began excavation of the northbound tunnel on July 9, 2014,[12] and reached Roosevelt station on March 17, 2015.

[16][self-published source][17] TBM #2 (formerly Pamela) began excavation of the southbound tunnel on November 20, 2014,[18] and reached Roosevelt station on July 13, 2015.

[20] Additional repairs to TBM #2 needed to complete the final (southbound) tunnel segment would have potentially delayed the project's opening if the five months of float time were exceeded,[21] leading JCM Northlink to finish the final segment with TBM #1.