Flooding in the Midwestern U.S. collapsed a railroad bridge and sent water surging around a dam Monday after days of heavy rains that have forced hundreds of people to evacuate or be rescued from rising waters. The flooding brought additional misery to parts of Iowa, Nebraska, South Dakota and Minnesota during a vast and stubborn heat wave. In some communities hit by flooding, the temperature Monday afternoon approached 100 degrees Fahrenheit. More than 3 million people live in areas touched by flooding, from Omaha, Nebraska, to St. Paul, Minnesota. Storms dumped huge amounts of rain from Thursday through Saturday, with as much as 18 inches falling south of Sioux Falls, South Dakota, according to the National Weather Service.