Many of us have tried to forget the historic heat of Late June. Sadly, even Link trains had to reduce speeds. Areas south of the DSTT ran as slow as 20mph and caused delays of 3-10 minutes. This surprised me: elsewhere, Light Rail often operates in temperatures well in excess of Late June’s. ST’s John Gallagher explains:

There are basically two things going on. One is that extreme heat can cause the rails to expand and change shape. The other is that the turnbuckles that keep the overheard catenary wires taut can expand, causing the wires to sag a bit. Out of caution, we operate Link at lower speeds when it’s very hot to ensure that neither of these problems interfere with service should they occur. 

Mr. Gallagher says that ST has already added air conditioning to substations to make them more resilient. New track extensions include a spring system on the overhead wires to replace the balance weights on the original track, which should improve heat resistance. He adds that ST will conduct a review to see if there are other changes necessary for a warming world.

Without overreacting to a single instance of record heat, all trends suggest that there are more and more extreme heat events coming, and ST should look to mimic systems like Phoenix that already deal with those conditions.

17 Replies to “About that heat dome”

  1. They do the same thing in Portland, except the speed restrictions for Max cut in a little sooner than here. But glad to see that ST is making incremental improvements. This is what an agency will typically do as they gain operational experiance.

  2. What does Phoenix do to solve this problem. I’m guessing summer will happen every year from here out and I don’t mean that as snark.

    1. while high temps do contribute to (accelerate) material failures, it’s not so much the specific temperature as the range that is important here

      that said, Seattle temps run pretty mild, so there must be plenty of places that deal with tensioning over a similar range

  3. Phoenix did detailed design studies to give waiting riders relief from direct sunlight at stations. It’s more than planning for just heat expansion.

  4. Had some fun digging through Phoenix’s design guidance for its light rail:

    and here’s Sound Transit’s, for reference:

    Phoenix’s design manual has a much more in-depth section on HVAC design than ST’s, and calls for a summer design ambient temperature of 112 degrees F, and a winter design ambient temperature of 34 F, and calls for other systems to operate between a temperature range of 25 F and 135 F. For the US’ hottest operating light rail system, it makes sense for them to put significant guidance towards managing heat in their system.

    Meanwhile, Sound Transit’s design guidelines (updated May 2021) call for a minimum temperature of 0 F and a maximum temperature of 105 F. The rail is assumed to have a temperature 35 F higher than ambient when exposed to sunlight, so is designed to be stable up to 140 F, but our last heat wave of 117 F would mean a track temperature of 152 F, well above the design parameters. Interestingly, the upper limit of catenary contact wire auto-tensioning is 130 F, and the lower limit is 5 F. I’m not sure what the expected heat-absorption factor is, but it seems that no one was expecting to build for a 110+ degree weekend.

    It seems to me that we’re going to be stuck with slow trains during high heat until there is significant track work to reset the neutral temperature of the rails to match changes in climate.

    1. Great work, Nathan! Understanding nuances of design standards is important. Thanks for digging!

      I guess my follow-up questions are:

      1. What would it cost to upgrade the current system for higher temps? Should ST install inexpensive passive adjustments like shade fencing to keep tracks in less direct sunlight?

      2. Is any project currently under construction able to be easily adjusted with a minor contract modification (time and money)?

      3. Should upgrades to design standards need to be made before the next generation of projects go to final design?

      1. It seems to me that there has already been enough internal discussion at ST to warrant updating the design guidance to include additional compensation for catenary wire (springs instead of weights, as discussed by Mr. Gallagher).

        Given that the 30-year climate normal temperature average has increased about 1.1-1.5 degrees in Seattle (, and we’re going to see more extreme weather, it would seem prudent to update the Climate Criteria to reflect those changes. Even an update to a maximum ambient temperature of 110 or 115 would seem prudent, but I have no idea what the cost of maintaining track and appurtenance operation from 0 to 115 degrees would look like. Adapting the existing track would probably raise questions of whether the metallurgy could even handle it. Also, adding shade to the track would have the problem of snow loading during extreme winter events, which are not uncommon.

      2. Shade fencing could easily have a foot of exposure at the bottom. It could be temporary or seasonal. I don’t see snow as a fatal flaw — but just a consideration. Heck, an overhead sloped canopy would deflect the snow!

      3. 105+ heat was only six or so days this year, and people were far more concerned with finding air-conditioned spaces than whether Link trains were temporarily slowed. We should make Link more heat-resiliant medium-term, but that doesn’t have to be rushed when so much else is going on right now. ST is busy finishing Northgate, Redmond, Lynnwood, and Federal Way by 2021-2024 and can’t change construction parameters at this late stage, but it will have more time after that to retrofit the tracks and bump up the ST3 standards. ST3 construction won’t start in earnest until ST2 is finished and all ST1/2/3 tax streams can be directed to ST3. (The last two Federal Way and Redmond stations are technically in ST3, but they’re part of the revised 2024 schedule so I’m lumping them under ST2.)

      4. Link is already extremely heat resistant. Unlike MAX, the track is held in place by concrete almost everywhere. Sun kinks typically happen in ballasted track, not concrete encased track. Only place I think where ST used ballast is along the SoDo busway, and at the shops building.

        They might need to adjust the counterweights on the catenary from summer to winter tension or something, but really Link is pretty solid.

      5. @G in P,

        Yep. Not really much of an issue for Link. It’s pretty solid.

        Additionally, approx 1/3 of the system is underground where the temperature is pretty darn stable. Not an issue there at all!

        Biggest problem with heat is OCS sag. Portland went with a lower cost tensioning system knowing full well that on hot days they would suffer sag and have to slow the system. But there is nothing wrong with that, it’s just a design trade – are the savings worth the occasional inconvenience?

        Portland made one decision, ST made a different one.

        ST went with a somewhat more robust tensioning system, but it can still suffer sag when the temps exceed design assumptions, but there is nothing inherently wrong or unsafe with that. And if it becomes too much of a problem they’ll probably make some tweaks.

        Was talking to a worker today at the Roosevelt Station. They are taking down the barricades! They load them up and take them away on Friday!

        Lots of great improvements coming with rail transit over the next 4 years. Pretty darn exciting times.

      6. Portland actually has constant tension catenary in most places. However, the counterweights are set for a low minimum temperature.

    2. There were plenty of Trolleybus problems as well. The overhead copper wire sagged and poles dropped. I saw it happen in a couple different areas. But the difference was, the new busses can go in to battery mode and drive through it. Then put the poles back up. Seven years ago they couldn’t. They had no battery back up. That might be one reason the problems were not as apparent to the public as Link’s issues.

    1. I used to do it regularly at my old apartment before a friend moved out of state and gave me her AC unit. That place was regularly 10+ degrees higher indoors than the stated highs for a given day, and could not vent heat for crap. Somehow it was also crap at retaining heat in the winter, too.

      So I’d either ride the F back and forth for several hours (closest major route), or head into the city and wander around because actual building shade, ride Link end to end sometimes. I couldn’t do that now, even without the pandemic.

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