Why our trains slow down when it heats up

Jun 23, 2015

Many of you have asked why our trains slow down when it’s hot outside. We know extra delays can be frustrating, but there are two important reasons why our speeds go down when temperatures go up—science and safety.

Update: How we’re minimizing heat delays on MAX

Like in other cities, the MAX light rail 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 have a hard time adapting.

Steel and copper expand in the heat

In the case of extreme heat, the rails (made of steel) and the overhead power wires (made of copper) expand.

A one-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.”

Additionally, the overhead power wires may also expand. 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. (But sometimes, it gets so hot that the counterweights touch the ground and the wire starts to sag anyway!)

At 90+ degrees, operators slow down for your safety

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 even more.

When temperatures hit the 90s, trains traveling in speed zones above 35 mph will need to run 10 mph slower. This will affect segments of all MAX lines and may cause minor service delays.

At 95 degrees, WES Commuter Rail trains must also run slower—no more than 30 mph—to ensure safety. This can cause up to 30-minute delays.

If temperatures climb above 100 degrees, MAX trains cannot go faster than 35 mph. Delays of up to 15 minutes should be expected.

It’s hot out there—so prepare for the heat and stay hydrated!

Brian Lum

Brian Lum

Web & Social Media Coordinator

I'm here to help tell our story, and to share the interesting things I find along the way. When I'm not here, you'll find me out riding my bike and taking pictures.

lumb@trimet.org | All posts

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