Seattle’s light rail is being made a one-size-fits-all solution that doesn’t (Part 1)

Tukwila International Blvd Station (Photo: Jason Lu)
Tukwila International Blvd Station (Photo: Jason Lu)

With light rail expansion and planning well on its way, things look positive for rail transport in the Puget Sound Region. The momentum and demand for rail-based transport in the region appears higher than ever, with residents beginning to realize that our current transport network is simply inadequate for the growth rate in this region. When a single fish truck can bring the region to hours of standstill, transport alternatives cannot come soon enough.

However, as with every major project, there is always a time to step back and once again look at the big picture. What type of transport objectives are we trying to accomplish? What kind of connections and services do we need?

But here is the biggest question that we need to answer before Sound Transit 3: What exactly are we building right now?

The simple answer is, of course, light rail. The more complicated answer is that we are building a regional light rail network.

And that could be a problem, because light rail vehicle technology is not intended for regional services. If Sound Transit pushes these vehicles to compete with cars between Everett and Seattle, or Tacoma and Seattle, it will have to find a delicate compromise between competitive travel times and travel time reliability. Let’s discuss why.

Vehicle performance matters in a regional context

In an urban environment where station spacing is generally less than a mile, top speed has very few benefits. The top speed will never be reached and travel time savings will be negligible.

Sound Transit, however, wants its rail infrastructure to go far beyond an urban environment. Its infrastructure is decidedly a suburban rail system outside of Seattle, with distances between stations ranging from 1.5 miles to over 2 miles. In fact, Rainier Beach to Tukwila International Blvd spans a massive 5.6 miles. If the network is extended to Everett and Tacoma, Link effectively becomes not just a suburban network, but a regional one.

These are network characteristics where higher top speeds make an impact by providing some travel time savings, but more importantly, travel time reliability.

Between Roosevelt and Lynnwood, a Link vehicle with an acceleration of 1 m/s^2 (a conservative value lower than the 1.3 m/s^2 prescribed  by Kinkisharyo), deceleration of 0.9 m/s^2 and a top speed of 58 mph can make the trip in just under 17 minutes at the very fastest, including 30 seconds of dwell times at each of the six stations. Real-world operations are likely to be slower, as this is using simplified calculations that assume constant acceleration and deceleration.

For comparison purposes, a typical heavy rail German S-Bahn BR 423 rolling stock with a top speed of 87 mph, but the same acceleration and deceleration, can theoretically travel between Roosevelt and Lynnwood 2 minutes faster. It could save an additional 2 minutes between Rainier Beach and Tukwila (although realistically, the current infrastructure doesn’t allow for higher speeds through the entire section).

Travel time savings with a faster and higher-accelerating train will only accumulate as the line grows farther out from Seattle. A half minute to a minute saved between each suburban station will add up significantly over the 60-mile regional line that Sound Transit proposes to build.

Higher speeds may benefit the timetable, but more importantly, they provide an opportunity to recover from likely delays incurred along the regional line without increasing travel times on the time table. Deutsche Bahn suburban trains use this technique to maintain punctuality in outlying areas, where the top speed is increased from the normal 120 km/h (75 mph) to 140 km/h (87 mph) to recover from delays.

There are many technical reasons for why Link’s infrastructure and vehicles in its current form may have difficulty achieving higher performance. Generally, light rail vehicles present some design challenges not present in conventional rail and require more mitigation to achieve higher performance. This will be explained in a future article.

Summary

Our to-be regional train is limited by the design nature of our vehicles and infrastructure to freeway speeds at 58 mph. Sound Transit can run these trains as fast as possible to provide competitive travel times against cars, but they will be less likely to recover from timetable deviations accumulated along the line.

In a line as long as Link’s spine will become, any delay can create operational headaches for both Sound Transit and riders, especially when the trains will already be pushed close to its performance limits. With East Link also using the tracks between Northgate and International District, delays will create problems when trying to coordinate shorter headways between the two lines and may even cause delays elsewhere in the network. The solution, of course, is to pad the schedule and increase normal travel times, making them  less competitive with cars.

Whereas most operators such as Deutsche Bahn operate regional services with faster heavy rail vehicles capable of providing both travel time savings and reliability over long distances, Sound Transit is trying to apply light rail.

This is an area where light rail may not be the most effective and the reason is simple: It is designed for an urban environment. It simply doesn’t provide the right characteristics for both fast and reliable regional services.

In Part 2, we will also discuss some possible issues of increasing capacity on Link using 4-car trains.

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