The kind of screening equipment used to check passengers at airports can't be used because it's too slow for 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. The next-largest systems are Chicago, Washington, Boston and Philadelphia. San Francisco has the largest system on the West Coast.

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 40 people.

In Madrid, 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.

The Transportation Security Administration was created by Congress after the Sept. 11 attacks to oversee security of all transportation modes.

Senate Commerce Committee Chairman Ted Stevens, R-Alaska, is among those in Congress who say the TSA has focused too heavily on airport security and has not done enough to protect trains and other forms of transportation.

The TSA missed a Dec. 31 deadline to submit to Congress a comprehensive security plan that covers all transportation modes.