Government top security officials said they are upgrading subway and rail defenses against terrorist attacks throughout the country, but a USA TODAY examination finds gaping holes, including many that may not be possible to plug. The holes in security leave travelers more vulnerable on the more than 4 billion trips they take by subway and rail each year than in the sky, where airlines carried fewer than 700 million passengers from U.S. airports last year. Six terrorist plots targeting U.S. subway and rail systems have been exposed since the September 11th attacks, and the systems remain a target, transit authorities, security expert, and members of Congress agree. They noted that about 15 million passengers board subway cars and trains unscreened each weekday. Mass transit systems are much less secure than the aviation sector or certain key government buildings, said DHS’s former inspector general.