OMAHA, Neb. — 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 mileswest of Omaha, was evacuated because of the smoke, and U.S....