Twenty-four people were injured and up to 10,000 were evacuated at Avon, Ind., after about 17,000 gallons of a toxic chemical spilled at a Conrail yard when steel slipped from a flatbed car and ruptured the tanker coupled behind it. Tom Drake, assistant chief of the Brownsburg Fire Department, said that people were evacuated for about 12 hours from a two-mile by three-mile area. Estimates of the number of evacuees varied from 2,500 to 10,000. The chemical was identified as acetaldehyde, a colorless liquid used to form other chemicals. Officials said its fumes are flammable and corrosive.