The 10-station line will be a key link in the region's public transport network, connecting the University Link light rail extension, which is due to open later this year, with Sounder commuter rail and Amtrak inter-city services at King Street station. The also serves business districts such as Capitol Hill, First Hill and Chinatown International District, as well as the Swedish Medical Center and Harborview Medical Center, Seattle Central College and Seattle University.

Construction began in April 2012 and the line was originally due to open in 2014, but the completion of the $US 134m project was delayed by the late delivery of the line's fleet of seven Trio Type 121 bidirectional low-floor LRVs, which were supplied by Inkeon, Czech Republic.