In addition to a dense network of bus routes, the cities of Seattle and Tacoma in Puget Sound, Washington are served by a rapid transit rail system named Link Light Rail. Designed, built, and operated by Sound Transit, the region's mass transportation agency, the system is currently composed of two lines. The Tacoma Link provides streetcar service in downtown Tacoma, while the Central Link light rail line operates between the University of Washington and the Seattle–Tacoma International Airport. The current system, which covers a total of 27.8 kilometres, is now being extended to connect the Seattle neighbourhoods of Northgate, Roosevelt, and the University District.

The final tunnel segment (U District to UW southbound tunnel) will be dug by TBM 1 (formerly Bertha). TBM 2 was supposed to finish it, having already done the previous two southbound segments, but ran into rocky soil north of U District station in December 2015.