Improving SODO Station with Cross Platform Transfers

Cross Platform station diagram from Urban Transit book

Cross-platform interchanges allow much easier transfers between two lines heading in the same direction. Implementing a cross-platform interchange station at SODO station could help connect Line 1 and 3 much better. Though there are some complications to building a cross-platform interchange.

Split Spine's Many TransfersLink Connectivity pattern with the 3 patterns as well as maintain regional connectivity" rightmost pattern from The Urbanist.

The extensions of Link Line 1 north to Everett and south to Tacoma will cause quite long travel times both for passengers and operators. Sound Transit plans to split the spine" after the addition of Ballard Link Extension to send Line 1 trains from SeaTac north to Ballard instead and the newly split Line 3 trains from Everett terminating south at West Seattle instead.

Westlake Station DiagramMidtown Station Diagram

One moderate problem with the new split is that the most popular segments from Northgate to Westlake and Rainier Valley to SeaTac Airport will be split and require relatively annoying transfers. One would have to transfer either at the deep Westlake Station with the 5~6 story transfer up and down around 8 escalators or at Pioneer Square Station with the block to walk across.

(This article will skip talking about Line 2 transfers and focus on Line 1 to Line 3 transfers. Please read Chinatown 5th Avenue Shallow Diagonal Station for Line 2 transfers.)

SODO Station SODO station diagram from https://westseattlelink.participate.online/sodo-station.html

SODO station uniquely has the 1 and 3 Line meet in parallel rather than at a 90 degree angle. Additionally the station is relatively cheap to build and expand, unlike Westlake or Pioneer Square station that are both underground.

The pink lines depict pathways to the platforms from the station entrance, as well as internal transfer pathways between the 1 Line (Ballard/Tacoma) and 3 Line (Everett/West Seattle) from Sound Transit

The currently proposed second SODO station requires transit riders from Line 1 to Line 3 transfers to take the escalator up to the mezzanine and back down on the other side.

SODO station with cross platform interchange. SODO diagram from Appendix J and annotated with Line 1 and Line 3. North is to the right

A same-direction variant would instead have the new Ballard-bound tracks and station built on the outer side. This would allow riders from Tacoma, SeaTac and Rainier Valley on the northbound Line 1 to easily walk across the platform to continue on the northbound Line 3 to Northgate, Lynnwood and Everett. The same would be possible in the reverse direction, just on the west southbound island platform. Transit riders from Ballard to West Seattle would also be able to easily transfer at the west platform.

For an American example (shown below), MacArthur BART station allows for same direction transfers with two island platforms and four tracks.

MacArthur BART station from Wikipedia with annotations.
Facing north with west island platform to the left and east island platform to the right

The outer tracks serve the Red and Orange lines while the inner tracks serve the Yellow line, allowing for easy transfers in the same direction. For example, someone heading south from Walnut Creek on the Yellow line to Fruitvale can easily transfer to the Orange line to San Jose on the western island platform.

Connecting to Ballard ConcernsWSBLE Existing, Interim, and Full Build diagram

One major complication with this alternate configuration is that southbound line from Ballard to SeaTac would be west of the line routed to from West Seattle to Northgate. While the currently proposed southbound Ballard to SeaTac line is to the east of the West Seattle to Northgate line. The new southbound tunnel would then need to be built west rather than east of the existing downtown tunnel.

If moving the new tunnel site is too complicated, an alternative simpler way to build the crossover stations would be to have the two inner lines crossover at grade both north and south of the SODO station. This would allow for a same-direction crossover station with the same tunnel plans, though the crossovers would force northbound Line 1 trains to wait for southbound Line 3 trains.

Appendix J drawings, page 16, L50-GSP118

The currently preferred alternative SODO station will already relocate the existing SODO station slightly south-east so Sound Transit will already be rebuilding the station. Secondly the new SODO station will already have a scissor crossing to the north of the station so all that would be needed is an additional scissor crossing to the south of the station. Sound Transit and SDOT are already planning on elevating Lander Street at that rail intersection so the road intersection wouldn't pose a problem for implementing a scissor crossing.

Extra Notes Cross Platform Diagram from wikipedia

There also exist more complicated variants for cross-platform interchanges in both directions. These allow easy transfers in all directions. A couple of these exist in Asia but are generally unnecessary for Link light rail given the relatively few riders transferring to and from West Seattle. Another downside is that the train would need to crossover twice and there would be a second south of SODO" station.

Conclusion

A cross-platform interchange at SODO station would allow for easier transfers for transit riders from SeaTac via Rainier Valley to Northgate and beyond. Riders wouldn't need to move up or down any levels and could easily walk across the platform. While there are some slight construction concerns, most of them could be mitigated.