Railway tracks are set on fire to ensure safety running for Metra trains 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. (Xinhua/Wang Ping)