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North Texas transit authorities slowed down trains when temperatures soared. Blame it on science

US DOT and John A. Volpe National Transportation Systems Center
When it gets too hot, train tracks expand out. These "bucklings" can be dangerous and cause derailment.

Dallas's record high temperatures have forced local transit authorities to reduce train speeds. Blame science for the delayed arrivals and departures.

At least two local transit authorities have reduced train speeds.

Trinity Metro reduced their speeds to 40 mph for three days (July 18-20).

And Dallas Area Rapid Transit (DART) reduced their train speeds between 1 p.m. and 9 p.m. on July 19 and 20 after National Weather Service predicted extremely high temperatures.

DART's trains traveled at 30 mph or less, compared to their usual maximum speeds of 65 mph. Officials warned that these speed restrictions would cause 10-to-20-minute delays for passengers.

Why do trains have to slow down to accommodate the heat? Let’s go back to Physics 101.

When temperatures rise, most metals — in this case steel — expand. Increases in temperature make the molecules in the metal vibrate. And that creates more volume.

DART officials say that their tracks are continuous welded rails with no gaps in between segments. Rather 1/4-mile long segments of metal are welded together at the ends. This design may be considered faster and smoother but it means that expanding steel has nowhere to go but out.

Stefan Dancila, an associate professor in the Department of Mechanical and Aerospace Engineering at the University of Texas at Arlington, said that extremely hot temperatures can cause serious complications.

“As the temperatures continue to increase, that creates some stress in the rails, and it causes buckling,” he said. “It causes them to stop being straight and to become curved.”

The phenomenon that Dancila described is called a “sun kink” — the rail expands out and creates a risk of derailment.

The heat similarly may affect the wires above the trains that provide power. The metal wires expand in extreme heat, which can make them sag.

Reducing a train's speed cuts down on the heat it creates. The faster a train goes, the more energy and heat it develops on the tracks.

Gordon Shattles, DART spokesman, said the track temperature was about 150 degrees Fahrenheit on August 20.

Slowing down the speed allows the operator to look out for buckling that could derail the train and prevent it in the first place.

DART is back to its normal speeds for now. Shattles said that they are monitoring the weather conditions on a day-to-day basis. It is possible that the train speeds will go down if temperatures go up again. He says that riders can register for DART alerts through their website for any updates related to train times.

Trinity Metro declined to comment unless provided written questions in advance.

Gloria Farris covers Fort Worth and Tarrant County. Got a tip? Email Gloria at

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Gloria is a News Reporter at KERA covering news from the Fort Worth and Tarrant County area.