Silver Spring, MD (AP) - A U.S. Postal Service worker was arrested Saturday in a bomb threat that forced the evacuation of two charter buses and the closure of the Capital Beltway for 90 minutes. The woman, whose name was not released, called a passenger aboard a charter bus carrying other postal workers from Merrifield, Va., to Atlantic City, N.J., and said there was a bomb aboard the bus, said Sgt. Russell Newell, a Maryland State Police spokesman.