The ST3 Sounder plan is still not very clear

Oran Viriyincy/Flickr

Ever since voters first had a look in 2016, the exact plan for South Sounder expansion in ST3 has been vague. Key elements are subject to negotiation with BNSF, who owns the track between Seattle and Tacoma. However, staff briefed the Sound Transit System Expansion Committee last Thursday on the recommendations they've been able to form since the last report in September, in the form of a draft Strategic Development and Implementation Plan.

Rider feedback is what one would expect: they would like trains to be reliable, less crowded, have the stations be nicer, and have more trips. Notably, there was more excitement about trips adjacent to current trips (in the peak, the shoulder of the peak, and evenings) than opening up entirely new times of day or weekends.

Staff is recommending progress on every axis of Sounder expansion (stations at Tillicum and Dupont in 2036 are already baked in the cake). They would make gradual station improvements over the next 20 years, especially at King Street Station where volumes are highest.

A rudimentary ridership analysis suggests that ridership demand would begin to exceed the current number of seats in the mid-2020s, dip when Link to Tacoma opens in 2030, and then exceed capacity again in the mid-2030s. ST can keep this problem at bay by going from 7- to 8-car trains around 2023 and 10-car trains around 2028.

As only Seattle and Tacoma have platforms long enough for as many as 8 cars today, from 2023-2028 rider education would have to reserve the 8th car for Tacoma or have people enter from the 7th car. The 2028 increment would include projects to expand all of the platforms to 10 car lengths.

There was little new information about additional daily trips. ST Express data suggests that peak trips are roughly 5 times more popular than off-peak trips, but there are constraints everywhere. Simulations suggest that bringing headways lower than 20 minutes would cause delays to ripple across the system. Freight volumes are much higher afternoons and evenings, where there would be genuinely new trip patterns and require fewer required trainsets, but also the most resistance from BNSF and a low cost-benefit ratio. I got the sense that the least bad option, in the staff's view, would be to add trips in the peak shoulder.

The briefing charts suggest, without much emphasis, that these negotiations might wrap up as soon as 2021.