Ludlow firefighters and Kenton County emergency workers were dispatched to a railroad tank car that was leaking compressed liquid carbon dioxide near Sandbank Road about 6 a.m. Tuesday. Residents in the area were initially advised to stay indoors. Officials later determined that the substance was nontoxic and the advisory was lifted, said Ludlow Fire Department Capt. Matt Chastain. He said a pressure-relief valve on the tank car apparently became stuck open, resulting in the leak.