Sound Transit 3 is the biggest investment in pedestrian mobility the Pacific Northwest has seen since the coming of the railroads in the 1890s. Like what that generation built, the capital projects we’ve committed to build will be around for decades. We can’t know with certainty what the future holds, but for reasons of both climate and mobility, it seems clear that our city’s future will need to involve more high-quality electric transit. It’s therefore essential that we build the core of ST3 so as to maximize its future utilization and value.

The current ST plan involves constructing a new tunnel from the International District to Uptown, constructing an elevated line onward to Ballard, and connecting the existing Rainier Valley line into this new tunnel. The Rainier Valley line is the only line slated to use the new tunnel (West Seattle Link and East Link will feed the current downtown tunnel), and the Rainier Valley Line’s surface-running section constrains it to a best-case headway of six minutes. The ultimate capacity of a rail tunnel depends on several factors, but a typical modern tunnel should easily be able to accommodate headways of three or four minutes.

If the new ST3 downtown tunnel is built without provision for an additional future line, there’s a real risk it will be permanently underutilized, moving ten trains per hour rather than the twenty or thirty it could — or at least, it would require major, disruptive engineering work to fix. Fortunately, making provision for future lines need not be expensive, as we show in Oran’s illustration above. A “stacked” station design with a pre-made bellmouth, as used by LA Metro at their Wilshire/Vermont station, requires slightly more excavation but allows for a new rail connection at minimal cost and disruption to ongoing service.

More after the jump.

We don’t have to go far to find an ideal candidate for a second north Seattle line. Aurora is the highest ridership corridor in King County Metro’s system, with RapidRide E handily beating Ballard’s RapidRide D, at 17,380 riders per weekday to 14,060. A future Aurora line could terminate in SODO, or it could continue south: perhaps as a line to Renton, or as the long-discussed “Duwamish bypass,” a fully-elevated fast line to SeaTac and Tacoma. Provision on the elevated SODO guideway will be much simpler, requiring only a small concrete stub to allow for a future track connection. LA Metro again provides an example with their junction at Crenshaw.

There are real fiscal pressures on the ST3 alignment, but what we’re proposing here does not require much money. What our proposal does require is consideration from ST’s engineers when the specifics of the South Lake Union and SODO alignments are designed. More generally, what this idea requires from all of us is concern and desire for a future of pedestrian mobility whose specifics are not known, but whose general direction seems certain. If you agree, we’ll be teaming up with other local transit advocates to ensure future extensibility is a squarely on ST’s list of requirements for their next downtown tunnel.