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The precautions are rooted in a fundamental vulnerability: extreme heat can warp steel tracks

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As the second heat wave this month scorches large swaths of the United States, the country’s rail networks are confronting a little-known strain: the limits of metal tracks under siege from the sun.

Rail operators are adjusting accordingly. A Pennsylvania transit agency has begun reducing speeds by as much as 10 miles per hour along certain corridors, while one operating in New York and Connecticut has imposed a 70-mile-per-hour cap — measures likely to trigger delays for commuters and summer travelers alike.

Elsewhere, at least one operator has turned to a seemingly low-tech fix: coating sections of track in white paint to blunt the heat’s intensity.

These measures are rooted in a fundamental vulnerability: steel rail can expand and buckle when met with high temperatures. “When that steel has nowhere to go, it can push sideways and create what we call a thermal misalignment,” Union Pacific chief safety officer Rod Doerr said in a recent statement.

On Wednesday, large sections of the Midwest, the Mid-Atlantic and the Northeast were under heat advisories, according to the National Weather Service, with temperatures climbing through the 90s and, in some places, into triple digits. The stifling conditions follow a record-breaking July 4 heat wave that swept across much of the country less than two weeks ago.

“If you will be situated within any of these areas forecast to experience extreme heat, stay hydrated, take frequent breaks, and limit outdoor activities if possible,” the NWS said Wednesday.