OMAHA, Neb. (AP) — Railroads work to continue moving freight regardless of the weather, but this week’s extreme cold is slowing traffic and forcing them to use shorter trains.

BNSF spokeswoman Amy Casas says that in subzero cold, the air brakes trains use become less effective because air flow is reduced. So railroads have to reduce the length of trains.

Chicago is a key hub where all major North American freight railroads meet and hand off traffic. With the extreme cold, freight railroads divert traffic away from the area when possible, and Amtrak cancelled all of its departures from Chicago on Wednesday.

Union Pacific spokeswoman Raquel Espinoza said shipments to Minnesota, Wisconsin, Illinois and Iowa are being delayed by between 48 hours and 72 hours.

The cold also takes a toll on rail and switches. Railroads use thousands of switch heaters to help ensure that the crucial devices will work.