WHAT'S NEXT MILLIONS of cargo containers full of toys, TV's and other consumer products stream into United States ports each year. But security experts fear the metal boxes could also be used to transport dangerous freight: terrorist weapons. Researchers are working on modifications to the rugged containers, adding electronic monitoring that can keep track of intrusions once the boxes are sealed at a factory and on their way by train, truck and ship. General Electric is testing a palm-size security device with a built-in microprocessor and radio. The device, which has been tried out on a handful of containers traveling...