Targets for Terrorism: Ground Transportation

An overview of ground transportation systems and their vulnerability to terrorist attacks.

Last updated July 13, 2006

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Could terrorists attack ground transportation in the United States?

Yes. Since September 11, U.S. authorities have issued several general warnings of possible terrorist attacks on parts of the ground transportation system, including subways, railroad trains, and bridges. Unlike airlines, where security checkpoints screen passengers and luggage, mass transit options like subways, passenger trains, and buses, are designed to be easily accessible and are therefore harder to protect. Ground transportation systems—which often include enclosed spaces packed with people—could prove tempting targets for terrorists.

How might terrorists attack ground transportation?

Experts say the most likely sort of attack on U.S. subways or buses would involve setting off conventional bombs; the materials and know-how are readily available. Nor do experts rule out the sort of suicide bombings that have targeted Israeli buses. Less likely but far more devastating scenarios involve the release of a chemical agent such as sarin gas or a biological agent such as anthrax or smallpox into a subway system. Terrorists could also derail a passenger train or blow up a bridge or tunnel, killing many people and crippling a city’s infrastructure for months or even years.

Have terrorists ever targeted ground transportation outside America?

Yes, often. The most recent include the March 2004 al-Qaeda bombing of rush hour trains in Madrid, killing 191 people, the July 2005 London tube bombings (three subways and one bus were bombed), which killed fifty-two people, and the July 2006 attacks on Mumbai’s commuter rail that killed more than 200.

Terrorists abroad have used guns and conventional bombs to kill and injure civilians in subways, trains, and buses for decades. Palestinian suicide bombers have repeatedly blown up buses in Israel, and the Irish Republican Army (IRA) repeatedly attacked the London Underground and British passenger trains. In the mid-1990s, Algerian extremists from the Armed Islamic Group (GIA) set off bombs in the Paris subway. Closer to home, terrorists linked to al-Qaeda planned to detonate truck bombs in New York City’s commuter tunnels and bridges in 1993; other Islamist terrorists plotted suicide bombings in New York’s subways in 1997. The first attack on public transportation involving weapons of mass destruction occurred in 1995 when the Japanese doomsday cult Aum Shinrikyo released sarin gas into the Tokyo subway system, killing twelve people and sending more than 5,000 to the hospital.

Would biological or chemical attacks in subways be particularly dangerous?

Yes. Subway systems have relatively small enclosed areas that become packed with people at predictable times during the day. Also, air currents above ground, as well as those generated by the movement of trains through the tunnels, could spread germs or gases throughout a subway station and through ventilation systems to the streets above, leading to the infection of large numbers of people. Further, symptoms of exposure to some biological agents, such as the highly contagious smallpox virus, might not appear for a couple of weeks—long after victims had left the station and unknowingly infected many others.

In the 1960s, the CIA and the U.S. Army conducted a test in which light bulbs filled with microscopic particles were dropped into the New York City subway system to measure the possible effects of a biological attack. The study revealed that a similar type of attack using a deadly disease agent such as tularemia would have infected as many as three million people, most of them above ground.

Would it be difficult to carry out a biological or chemical attack on a subway?

Yes. Anthrax and especially smallpox are very difficult to obtain, and special scientific and technical skills are required to disseminate them effectively as a weapon. The same is true for deadly chemicals such as sarin or VX. Even Aum Shinrikyo—which had money, scientific expertise, and professional-grade labs—did not kill large numbers of people in its attack on the Tokyo subway.

Still, experts warn that less deadly toxic industrial chemicals—such as chlorine, phosgene, and cyanide—are easier to obtain and could also be used to kill and injure people on transportation systems.

Have subways increased security since September 11?

Somewhat. Many metropolitan transit agencies have increased both undercover and high-profile police patrols and have refined their emergency response plans to consider terrorism. In Washington, D.C., subway trash receptacles are being replaced with bomb-resistant cans. The Washington Metro has also conducted smoke tests to study air flows within the subway system and has installed a chemical detector in one subway station to provide early warning of an attack. In New York, suspicious packages are now regularly investigated and X-rayed and passengers’ bags are subject to random searches. But experts say bringing airline-style security to U.S. subways would be virtually impossible, and the above measures would be useless against suicide bombers.