Explainer: What is a heat restriction, anyway?

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(San Francisco—Casey Miner, KALW News) It's been downright sweltering in the Bay Area over the past few days, and here at KALW News we’ve been enjoying the sudden summer weather by taking our laptops outside. But the heat hasn’t been so kind to everyone.  Both BART and Caltrain are experiencing heat-related equipment malfunctions that have led to long delays: computer failures on BART forced conductors to operate trains manually on Tuesday, while Caltrain cars had to slow down significantly due to “heat restrictions” on speed. From the San Jose Mercury News article on what happened:

“On the Caltrain tracks, trains were being slowed down from their top speed of 79 mph, according to a release from spokeswoman Christine Dunn, who explained that in extreme heat, tracks become soft and can be damaged by the weight of the train.”

Tracks going soft in the heat? Sounds kind of dangerous, right?

As it turns out, though, it’s actually a fairly common occurrence, as are the heat restrictions that accompany it. Caltrain’s tracks, like many other rail tracks across the country, are made of something called continuous welded rail (CWR), a kind of steel track that has very few connecting joints and thus makes for a smoother ride.

The flip side of fewer joints, though, is that when it gets really hot – 100 degrees Fahrenheit or hotter – the rail expands, weakens and becomes more likely to buckle. Fast trains put additional stress on the tracks, so slowing them down helps alleviate the pressure and avoid buckling.

“This got everybody’s attention, but it’s not an unusual thing,” said Caltrain spokeswoman Christine Dunn when I called her Wednesday. Dunn told me that Caltrain doesn’t keep records of how often it’s had to implement heat restrictions, but given the rarity of 100+ degree days in the Bay Area, one imagines it’s not too often. That’s a marked difference from the East Coast, where some Amtrak lines impose heat slowdowns several times a month in the summer.

Heat restrictions are temporary – CWR tracks contract to their normal size when the heat wave passes. So for better or worse, everyone’s commutes should be back to normal after this week.