Ask TriMet: “Why do MAX trains have to slow down in hot weather?”

“Ask TriMet” answers some of the questions we get frequently from riders. Have a question? Send it to and we may feature it in a future post.

MAX Yellow Line traveling to Expo CenterDuring last week’s heat wave, many riders asked us why MAX has to travel slower when it’s hot outside. In short, trains slow down in 90+ degree weather as a safety precaution because extreme heat can cause the rails (which are made of steel) and the overhead power wires (which are made of copper) to expand. Here’s a little more of the reasoning behind this safety measure:

When it’s 100 degrees out, a 1-mile stretch of rail in the MAX system may expand up to a few inches. This rail has to go somewhere, and when it gets too hot it can actually bend or lay over on its side! Our operators and controllers call this a “sun kink.”

The overhead power wires may also expand in the heat. Because copper expands more than steel, and because we can’t allow the overhead wires to sag, we have a system of pulleys with counterweights that tug on the wires to keep them tight. Sometimes, it gets so hot that the counterweights touch the ground and the wire starts to sag anyway.

Our operators have to watch for both sagging power wires and “sun kinked” rails when it’s really hot out. To be safe, they slow down to make sure nothing goes wrong. As it gets hotter, they have to slow down more.

So what does that mean for riders?

When temperatures rise into the 90s, trains traveling in speed zones above 35 mph will need to run 10 mph slower. This will affect segments of each MAX line and cause minor delays to service.

If temperatures climb above 100 degrees, trains cannot go faster than 35 mph, resulting in delays up to 15 minutes throughout the system.

Like in other cities, the MAX system is designed for the average temperature ranges of our local climate. When temperatures are at the extremes of that range, the materials in the system are sometimes unable to adapt.

Have a question? Send it to and we may feature it in a future post.

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