Less than a week ago Britain was remembering the victims of the London bombings that ripped through its transport network on 7 July 2005. The seven blasts in Mumbai's suburban railway network on 11 July 2006 eclipsed this event. This is not the first time that terrorists have targeted public transport systems; on 20 March 1995, the Aum Shinrikyo cult released sarin gas in the Tokyo subway, killing twelve and injuring thousands. On 6 February 2004, a rush-hour blast killed several people on a metro train in Moscow. The 2004 Madrid blasts entailed coordinated bombings in the commuter train system of Spain. Why do terrorists attack transportation systems? Why do they repeatedly strike such targets in which the innocent become victims? Do they feel they can garner headlines and media attention easily by sabotaging public utilities? Undoubtedly, the transportation systems are the strength of any nation; attempts to disrupt them are an effective assault. Besides facilitating travel and mobility for the people, a nation's economy hugely depends on the road and transit systems. Hence, sabotaging them is akin to killing two birds with one stone - terrorizing the population and causing economic damage.

Mass transit systems are preferred by terrorists since they present high concentrations of people, and provide ample scope for large-scale destruction. Ensuring effective security is difficult because of the high traffic volume. A large number of access points with few or no inspection procedures compound security problems. The lack of mandatory passenger identification for boarding these systems also makes mass transit systems preferred targets when compared with government or military facilities.

Mumbai has witnessed such attacks in the past. The 1993 bomb blasts ripped through its stock exchange, trains, hotels and gas stations. Mumbai is India's commercial capital and its rail network is the city's lifeline. Reportedly, about six million people travel on the city's suburban railway system everyday. It is one the busiest networks, providing a quick and cheap mode of transport, linking the city's ethnic, religious and social classes, comprising bank and government officials, corporate employees, and other blue and white collar workers. Tuesday's attack was an assault on the nation's economy. Similarly, the Madrid and London attacks occurred in large cities, during rush-hour, thereby ensuring maximum damage.

Clearly, transport systems have become the most favored targets for terrorist organizations. Trains are more vulnerable since millions of people use mass transit systems every day and there is also no certain way to defend buses and trains from terror attacks. Secondly, the sophisticated screening methods adopted to check passengers at airports cannot be replicated in railway or bus stations, as it is difficult to quickly check large number of people. Targeting trains creates anxiety among people and causes massive economic disruption. It is estimated that such attacks, on average, cause more than two-and-a-half times the causalities per incident when compared with other forms of terror attacks. In short, attacks on surface transportation are among the deadliest, ranking behind attacks on aviation and almost equaling fatalities caused by attacks on religious and tourist targets. In economic terms, an attack on transit systems disrupts trade, but the costs in managing the attack's aftermath are much higher than the actual destruction due to the terror attack.

There is also the interdependency factor. Modern technology is interdependent with regard to communications, electrical power, rail and fuel distribution. Failure in any one of these critical infrastructure brings life to a standstill. Attack on any one of them affects the others. For example, the attack on the Twin Towers reportedly damaged the central telephone board, thereby affecting several communication antennas. The vulnerability of infrastructure networks induces terrorists to attack electrical power networks, railway links, and oil pipelines.

With such attacks becoming more common, state governments should be more responsive to threats. Agencies like the police, intelligence bureaus and fire brigades should collaborate with the public transport staff, assist them in detecting potential threats, and take immediate necessary action to respond to an attack. At the same time, closer coordination is needed between the government, security agencies and the transport sector. Inculcating heightened awareness among passengers is a challenge; but this would help to deter terror incidents. Since the threat from terrorists to transport systems is insidious, vigilance and preparedness to avert this threat is the need of the hour.