The Northgate Link Extension Light Rail project connects Sound Transit’s University Link light rail segment to the Northgate business/retail center in Seattle, Washington. The project forms part of a system connecting four major urban centers to the existing Central Link (which extends from downtown Seattle to SeaTac airport). It consists of 4.3 miles (6.9 km) of double-track light rail, 3.5 miles (5.6 km) in twin bored soft-ground tunnels and .8 miles (1.3 km) in retained cut fills and elevated structures.  Precast segments were used for the tunnels.

The project also includes two underground transit stations (U District and Roosevelt), one elevated station (Northgate), a portal structure, and 23 cross passages. Both underground stations were designed as bottom-up excavations, and the cross passages are sequential excavation method structures. The project was constructed in an area characterized by dense, urban neighborhoods and complex subsurface and groundwater conditions, which required optimizing the location and alignment of the underground elements.

As the prime consultant, McMillen Jacobs provided project management, tunnel and underground structure design, estimating and scheduling, and geotechnical engineering services. The team included more than 30 firms, including key teaming partners WSP, KPFF Consulting Engineers, Hewitt Architects, and LMN Architects. In 2018, Northgate Link received the Tunnel Achievement Award for Project Excellence from Tunnel Business Magazine, in partnership with the 11th annual Breakthroughs in Tunneling Conference.

Completed in 2021, the Northgate Link Extension aims to increase light rail ridership, improve travel time, and add transit capacity to the congested Interstate-5 corridor.