A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday. (Photos: Xinhua)

A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday.

A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday.

A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday.

A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday.

A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday.

A US 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 cause traffic disruption. The biggest city in the US Midwest was hit by the polar vortex with minimum temperature reaching record low of minus 30 degrees Celsius on Wednesday.