Sound Transit is seeking public comment on a program of possible expansions to Sounder South. These are likely to include additional daily runs on Sounder and station platform improvements to allow 10-car trains to operate (up from 7 cars today). Sound Transit envisions a series of improvements rolling out through 2036, with planning on the first projects beginning in 2020.
The ST3 program included $934 million (2014 $) in Sounder South capital improvements to improve access and capacity. There is an additional $325 million to fund an extension from the current terminus at Lakewood to serve two new stations at Tillikum and Dupont in 2036.
Sound Transit owns the tracks south of Tacoma, but the tracks between Seattle and Tacoma are owned by BNSF. That means improvements and slots for added trains must be negotiated with BNSF. The 2016 plan was vague on what improvements would be made because it involved both a negotiation with BNSF and a lengthy examination of trade-offs in selecting options for expanded service.
Some of those trade-offs have come into view. Adding trips at peak hours would serve more riders than adding trains at other times during the day even though those other times are sparsely served. It’s also less disruptive to BNSF freight operations which are mostly outside the peak Sounder window.
Adding trains off-peak would require fewer new train sets, but need more infrastructure to avoid freight conflicts. The additional track time from BNSF would be more expensive for this reason. Late evening trains, because they are furthest from the existing window, would require the most infrastructure improvements.
Increasing train frequencies from the current 20-minute headways to 15 minutes would reduce crowding but also risk delays because of constrained movements at King Street.
Longer trains would avoid some of these problems, but also require purchasing more train cars and storage and rebuilding stations for longer platforms. That will be particularly difficult at Auburn where the east side platform can probably only be extended for eight car trains because of the proximity of the Stampede Pass rail junction.
Sounder South has seen steady growth with average weekday ridership recently reaching 16,500, up 30% since 2014. With additional runs added in 2017, there are now 13 trains per direction each weekday, along with occasional extra trains for events on weekends.
Some improvements are already in the pipeline. New parking garages are scheduled to open in 2022 at the Puyallup and Sumner stations. Two more garages at Kent and Auburn will open in 2023. Together, these add 2,150 parking stalls. The projects also include bus and non-motorized access improvements at those stations.