HEAT WAVE. (and MAX hot weather operations)

Question: Why do the MAX trains go slower when it’s hot out?

I’ve been waiting months to make this post.  Not kidding – I know this question comes up every year when Portland gets a heat wave, so in February I took this picture:

Weighted catenary, February 2010

The temperature that day was somewhere in the 50°s – notice how far down on the pole the weights are hanging.

And compare the weights to this picture, same location, yesterday when it was about 88°F:

Weighted catenary system July

Edit evening of 07-08-10:

Here’s a picture from earlier today when it was about 100°F.

Weighted catenary at about 100 FYou can see that the weights are down a few inches from where they were at 88°F, and much farther down from where they were in February

I asked one of the MOW guys to explain how this works to me a while back. The pantograph on the top of the train is spring-loaded to push upward on the overhead wire, so the wires need to maintain tension. Tension in the overhead catenary wires in the high-speed areas of the alignment is kept in balance by weights like these – as warm weather makes everything expand, the weights drop further down to maintain tension in the overhead and keep it from sagging. But there comes a point where the weights hit bottom in extreme temperatures. When that happens, any additional expansion from the heat will make the catenary wires sag since the weights can’t drop any further to provide tension. So the trains run slower to avoid damage to the pantograph (and overhead wire) since the overhead wire has gone slack and can no longer provide the necessary resistance against the pantograph. Trains will drop their speeds by about 10 mph in high-speed areas when it gets to be above 90°F, and drop the speeds even more when it goes above 100°F. Plan accordingly if you will be traveling by MAX train!

Sun kink is another concern in extremely hot weather.  I have no TriMet pictures of sun kink, so here is one shamelessly borrowed from the Iowa DOT:

Sun KinkSun kink in rails

I’m no physicist so I’m not going to attempt to explain the nitty-gritty science behind it. But basically the ballast & railroad ties can keep the rail in place with normal heat expansion and contraction. However, with extreme heat, the construction of the rail can’t handle the force of expansion, causing the rails to slide laterally. There is especially high risk of this happening with big temperature swings (very hot during the day, cool at night), so during the summer it’s not uncommon for there to be slow orders for trains in areas of the alignment at risk of sun kink.

Slow OrderOld pic – I don’t remember why this flag was out, but here’s a picture of a wayside flag designating a slow order – a train must be at the speed limit posted on the yellow slow order flag when the front of the train reaches the flag, and then proceed no faster than that limit until it passes a green wayside flag.

You may also remember earlier this year when expansion joints were installed in areas of the rail on the west side and on Interstate where sun kinks are likely to develop. These joints give the rails room to expand to reduce the likelihood of the rail buckling.

104 in busTemperature in one operator’s unairconditioned bus

And my own PSA/editorializing during the heat wave – be nice to your operators! You can get off of the 100°F+ bus when you get to your destination – they’re stuck on it for their whole shift! (and even the air conditioned buses and train cabs can get the whole greenhouse effect going on too – this is not the nicest time of the year to be a transit operator! I mean, they tell you on the news not to leave your kids or pets in a car even with the windows opened because the internal temperatures can top 100°F – what about your bus passengers & operators?)

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6 responses to “HEAT WAVE. (and MAX hot weather operations)

  1. What’s that ball thing on the second picture by the track?

    As usual, completely informative, had no idea that’s how it worked.

  2. As always, you are right on the money! :-) I am actually happy to be back at bus in the heat wave; I get more air sitting in my bus driver’s seat than I did in the rail cabs. :-)

    I tried to explain the way the catenary wire sags during the heat to some bus drivers the other day – they had never heard of the overhead wires actually sagging and that that is one important safety reason why the trains have to slow down.

    • I added another picture from a few hours ago when it was about 100°F – weights were down even more.
      I know that this comes up every year, and every year there’s the same customer service complaints that the trains are going slow.. personally I’d like to see a proactive campaign about this kind of thing (and winter weather too!) since it’s not a surprise that it will happen but it catches passengers off guard if they don’t expect it, and they never seem to.

  3. Yep, we’ve been discussing this over here at Center St. and I knew I should check your blog for answers! I love the sun kink photo.

    • Happy to help! I can’t take credit for the sun kink picture – I try to only post photos that I’ve taken but I have nothing to show what sun kink looks like. Which I guess is good, all things considered – the fewer repairs like that that need to be made, the better!

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