Part of the problem is that the kind of screening equipment used to check passengers at airports for explosives can’t be used in subways because it’s too slow and too expensive for public transport systems designed to quickly move large numbers of people. “Mass transportation systems will always be vulnerable to some extent if we want to keep them as efficient as they are today,” said Rafi Ron, president of the Washington-based transportation security consulting firm, New Age Security Solutions. About 29 million people take commuter trains and subways daily in the United States, with the New York City area accounting for about a third of the total, said Alan Pisarski, a Washington-based national transportation policy analyst. In the UK, the vast London underground carries 2 million people each day. James Carafano, a homeland security expert with the Heritage Foundation think tank, said trains are a tempting target for terrorists because they’re so predictable. “It’s very, very easy to do reconnaissance,” Carafano said. Some of the deadliest terrorist attacks since Sept. 11, 2001, have been carried out on subway systems, including Thursday’s attack in London that killed at least 60 people. The attacks there have been claimed by Al Quaeda, the Islamic terrorist group responsible for the 9-11 attacks. Al Quaeda also claimed responsibility for attacks on the Madrid subway, where a railway bombing on March 11, 2004, killed 191 people and injured thousands. A month earlier, an explosion ripped through a subway car in the Moscow subway during rush hour, killing 41 people.