The 23km line will branch off the existing north-south line at International District/Chinatown station in Seattle, crossing Lake Washington to serve Mercer Island, Bellevue and the Microsoft campus in Redmond.

The $US 3.7bn project is being funded with the aid of a $US 1.3bn Transportation Infrastructure Finance and Innovation Act loan from the United States Department of Transportation.

When it opens in 2023 the 10-station line will offer a 24-minute journey time between Bellevue town centre and Westlake station in Seattle.