Sunday, September 19, 2021

Minimizing Downtown Transfer Pain in ST3

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SODO Station and the SODO busway: Future home of West Seattle – southside train transfers
Photo by Joe Kunzler / flickr

When ST3 is built out, it will have two major downtown transfer hubs, at already-very-busy Westlake Station and International District / Chinatown Station. In all likelihood, transfers at these stations will not be fast, and transfer volume may be one of the factors setting minimum headway on the busiest line.

There will also be transfers going on that don’t have to happen at those two stations, and some that will certainly be happening at SODO Station. For this post, I will be focusing on the transfers that can and should happen in the SODO.

The suggestions below are probably not in alignment with the Level 1 options going forward, but there is still time to give some attention to SODO station-area details.

Cross-platform transfers at SODO Station and Stadium Station

The algorithm for how to make transfers quick in the SODO is actually pretty straightforward, and doesn’t require particularly clever engineering. It would, however, probably require tearing out some of the existing infrastructure that wasn’t built with ST3 in mind.

  • Have West Seattle to North Link tracks stay to the west. Have South Link to Ballard Link tracks stay to the east.
  • Stack center platforms at SODO Station and Stadium Station.
  • Offset the platforms from each other enough to allow tracks for each line to slide by each other vertically.
  • Have one of the SODO platforms serve cross-platform transfers between northbound West Seattle Link and southbound South Link. Have the other SODO platform serve transfers between northbound South Link and southbound West Seattle Link.
  • Have two of the tracks on the same line slide by each other vertically between SODO Station and Stadium Station.
  • Stadium Station would then be set up to have both northbound tracks serving one center platform, and southbound tracks serving the other center platform, enabling the quickest same-direction transfers between the two lines.

That covers pretty much all transfers that could be moved out of International District / Chinatown Station and Westlake Station, except for transfers that could be avoided through use of different through-routes than the current proposed basic operational plan.

Cross-line Through-routes

Having track through-routing between East Link and Ballard Link near ID/CS, and track allowing through-routing between North Link and South Link should be a high priority.

Ridership demand for these through-routes could turn out to be higher than that for East-to-North one-seat-rides and South-end-to-South-Lake-Union one-seat rides.

Regardless, having a split service pattern during peak hours, with every other East Link train going to Ballard and vice versa, and every other South Link train going to Snohomish County, would help reduce peak dwell-time congestion at Westlake Station and International District / Chinatown Station. Lowering dwell time at the bottleneck stations could enable higher frequency and higher system capacity in the future. Four extra cross-over track segments are a cheap investment compared to having to build a third tunnel.

To connect South Link to North Link smoothly and cheaply:

  • Assume the algorithm described earlier in this post has been deployed.
  • Have the West Seattle tracks pass each other vertically between SODO Station and Stadium Station. Keep the South Link tracks on their same vertical level between the stations.
  • Build two diagonal tracks from the South Link tracks just north of SODO to the West Seattle Link tracks just south of Stadium, with junctions to the main lines. These two cross-over tracks would run essentially stacked, but each staying essentially at their same height between the junctions.
  • These tracks will come in very handy for broken-down North Link trains to be able to directly access the SODO Operations & Maintenance Base along the South Link tracks.

West-Seattle-to-Ballard should have a smooth enough transfer environment at Stadium Station, rendering it better than alternating one-seat rides. Adding additional cross-over lines between Stadium and SODO would be vertically challenging. But trains from West Seattle Link will still need a reasonably-direct path to the SODO Base, to avoid tying up multiple lines due to a broken-down train. Fortunately, having the cross-over tracks between South Link and North Link would enable West Seattle trains to get to or from the maintenance base with just one turnback, at Stadium Station.

Future proofing: Duwamish Extension

If the West Seattle tracks stack up before entering SODO Station, they will need less horizontal space. Leave room at the turn into SODO Station for tracks to Georgetown and points south to junction with the West Seattle lines. Build those junctions into the track to avoid line closures if and when such a line gets built. These junctions in and of themselves ought not be expensive, but are getting tossed aside during the Level 1 alternatives narrowing process under the assumption that they are what is driving an expensive stacking of SODO Station. In reality, many operational considerations ought to lead to that stacking.

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