The shaky start of the Olympic System, and the focus on making piecemeal expansions to the extant lines finished upon its completion, has led to a notable lack of major capital invest in fully new line operations. The extensions to Redmond, Kirkland, Des Moines, Tukwila, and the Renton Landing were all done to tie existing lines into (or near to) emerging residential and commercial nodes or transportation infrastructure such as the Soundrail S-train system. After the Olmypics, new leadership in both Seattle and Olympia passed new, comprehensive transportation packages - to address intercity, regional and S-train rail rather than the subway, and STS placed focus on the bus network. By the time new bond authority was released to expand the network, a "circulator" streetcar in downtown and then streetcars up Beacon Hill and down Rainier Avenue were the choices instead, operating from the vicinity of the King Street Complex along Jackson Street rather than new rail tunnels for the Subway. The largest initiative, in fact, was to rebuild and expand many stations to integrate them into new developments adjacent, expand retail mezzanines, install modern escalators and other updates.
This changed with the 2015 agreement between STS and King County to develop two new lines starting in Issaquah, a suburb about 27 kilometers east of Seattle and a major, growing jobs and residential node. The first line, with stops on the south side of the I-90 freeway in downtown Issaquah, would then connect to the Eastgate neighborhood of Bellevue and the Sunset Mills redevelopment before splitting in two - Line 14 would then join the Downtown Bellevue line trackage, and Line 15 would continue west, towards Seattle and across the West Seattle Bridge, where it would split from the two current lines using that trackage into a new tunnel under central West Seattle, with stops at the West Seattle Triangle, Alaska Junction, and Juneau Street in a tunnel under California Avenue before extending down to the Washington State Ferries Fauntleroy terminal. Line 15 would thus connect the metropolitan area east-west, allowing one to ride from the last major suburb before the Cascades all the way to ferry services across the Puget Sound.
Controversy followed the choice to build this routing followed almost immediately. It was lost on few that Beacon Hill and Rainier Valley, two lower-income neighborhoods of the central city, were given streetcars that on some portions of the route were made to share the road with personal vehicles, while fully grade-separated Subway services were being sent to affluent areas such as Issaquah or West Seattle. Both Kirkland and Redmond wanted to be the end-point of the Line 14 route as well, to benefit from more frequent service, and downtown Bellevue would thus have a train every two minutes on the center of its trunk route.
STS eventually compromised with every other Line 14 train going to either Kirkland or Redmond rather than one or the other, and reworked their timetables to account for Line 14 opening earlier than Line 15 - on October 30, 2023, the train launched for the first time, with full revenue service beginning on November 3. Line 15 was more complicated - Seattle's frustration over the streetcar controversy made an issue and the tunneling under West Seattle was delayed in an eerie replay of the Olympic Lines, and the utility of extending service all the way to the ferry terminal and the necessary property acquisitions around Lenihan Park were raised. The line is currently expected to open in the spring of 2025, nearly eighteen months after originally planned, at which point riders disembarking from the ferry can ride with a single transfer to downtown Seattle, the airport, or the "secondary" regional downtown at Bellevue, or a host of other destinations.
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Beyond Lines 14 and 15, it is unclear what exactly is next for STS. A program to refurbish twenty stations by 2030 is in the works, as is a pilot for driverless trains currently underway on the Capitol Hill Circulator (Line 6) that is staunchly opposed by the transit operators union but supported by a number of transit development organizations. The only planned extension is building out Line 12 further into Kirkland from its current Rose Hill terminus, likely to be completed by 2030-31 once final route approval is completed. Extensions further north in Richmond from the Westminster Triangle and further south from Highline College in Des Moines are considered high-priority for study; a brand-new Line 16 running east-west on 85th street as a secondary "cross-city line" has been proposed, but is regarded as a difficult project to approve and construct. Some advocates continue to suggest that investing more in Soundrail regional services that would allow higher frequencies, larger trains and better interconnection outside of King County is the better program to pursue, rather than having the Subway take on some S-train adjacent features; others note that the Seattle Subway, soon with fifteen lines and 138 stations, is a remarkably mature "system" by international standards for a metropolitan area of its size, even without taking into account the Soundrail lines, and that further pedestrianization and bicycle infrastructure in a city considered the national leader in such efforts is more important than further lines that already cover most places where they could reasonably be needed.