OMAHA, Neb. (AP) — An explosion inside a shipping container at the world’s largest railyard prompted evacuations in western Nebraska Thursday because of the toxic smoke generated when one of the chemicals aboard caught fire.

Around noon, an explosion occurred inside an intermodal container on a railcar at Union Pacific’s Bailey Yard in North Platte, though it wasn’t clear what caused the explosion, railroad spokeswoman Robynn Tysver said. No one was injured, and no cars derailed.

Authorities evacuated everyone within a four-mile radius of the railyard, which is about 280 miles (451 kilometers) west of Omaha, was evacuated because of the smoke, and U.S. Highway 30 was closed between North Platte and Hershey. Interstate 80 wasn’t affected by the smoke. North Platte has a population of about 23,000.

The North Platte Fire Department said in a post on X that the evacuations were done because of the fire at the railyard involved “heavy toxic smoke.” Fire officials didn’t immediately respond to a call seeking more details.

One of the containers involved was carrying perchloric acid, which is used in explosives as well as a variety of food and drug products, Tysver said.

The railyard where the explosion happened covers 2,850 acres and stretches as wide as eight miles at one point. A few years ago, an eight-story tall observation tower called the Golden Spike Tower was built to allow people to watch thousands of railcars be sorted from one train to another on Union Pacific’s key east-west corridor.

Railroad safety has been a key concern nationwide ever since a Norfolk Southern train derailed and caught fire in eastern Ohio. That derailment prompted evacuations and calls for reform from members of Congress and regulators.

The National Transportation Safety Board didn’t immediately respond to an email message from The Associated Press asking whether they will send a team to investigate the explosion.