The sight of flames sprouting from railway lines in Chicago with commuter trains travelling along them looks dangerous, but it's all done deliberately in cold weather, like the current big freeze.
Flames were seen coming from the tracks of Chicago's Metra commuter rail system earlier this week, CNN reports.
A Metra spokesman explained the company isn't literally setting the tracks ablaze, but the flames come from gas-fed heaters that run alongside the rail lines and keep them warm, so the trains can keep running.
Metra also uses a tubular heating system and hot air blowers to heat up cold track, he said, adding that other rail systems in North America use similar systems to heat the tracks.
The system is used anytime it's below freezing, and this week with the wind chill taken into account, Chicago temperatures have plunged to minus 50 degrees.
Metra said it's safe to run the trains over the flames because the diesel fuel in the trains "combusts only with pressure and heat, not open flames".
Tracks are affected by extreme cold in two ways, Metra explained in a social media post.
The extreme cold shrinks the metal and the rails pull apart from each other. Heating the tracks with fire expands the metal until the two rails can be put back together again.
Railroad switch points can also become clogged with ice and snow in subzero conditions, so the heating system is used to unclog them.
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