CHICAGO - A U.S. Metra train runs on tracks on fire at Western Avenue station in Chicago, the United States, on Feb.1, 2019. Chicago’s railway system uses gas-fed heaters that run alongside the rails to keep them warm, in case frozen tracks causes traffic disruption.

 The biggest city in the U.S. Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday. Several local reports showed rail crews using fire to heat up railroad tracks to ensure trains can run smoothly and without incident.

The rail, which is used both for commuter travel and commercial cargo, is critical not only to Chicago, but also businesses across the United States. And when temperatures get well below freezing, the cold can cause bolts to erupt from the rails. Cold temperatures can also stop switching points on rails to work properly.