The devastating bomb attacks that ripped through four commuter trains and killed more than 200 people in Madrid on March 11 has led security analysts to focus on securing the world's railroad networks. Unfortunately, experts throughout the world agree that railroads cannot be completely protected from terrorist attacks, since the implementation of airport-style security measures is largely unfeasible.

Always eager to exploit vulnerabilities, terrorists have demonstrated a sustained interest in targeting rail systems. While a number of plots have been thwarted by solid intelligence gathering and aggressive police work, there is every indication that subways and trains will continue to be attractive targets for terrorists, who have been able to refine their tactics by studying previous plots. There is evidence, for example, that the Madrid bombers may have learned from the past successful efforts of Chechen terrorists.

Chechens have used nearly every conceivable tactic to inflict maximum damage: from suicide bombers blowing themselves up on commuter trains during the morning rush hour to planting bombs on railway tracks in the proximity of crowded stations. While last month's bombing of the Moscow subway, which claimed forty lives, attracted widespread press coverage, several other attacks in the rural southern areas of Russia by Chechen terrorists have received little attention despite the carnage they have caused.

Significantly, a now-dismantled Al-Qaeda cell that was based in Madrid and actively helped in the planning of 9/11 had several key links to Chechen extremists. In fact, Imad Eddine Barakat Yarkas, the now incarcerated leader of the cell, is accused by Spanish judge Baltasar Garzon of having recruited several militants to train and fight in Chechnya alongside Al Qaeda. Moreover, Yarkas coordinated fundraising efforts within the Madrid Muslim community for Chechen "freedom fighters."

Abu Qatada, a Palestinian cleric who Spanish authorities have described as "Al-Qaeda's spiritual leader in Europe," coordinated the Chechen fundraising from London. Yarkas frequently traveled to London to give funds collected in Spain to Abu Qatada, and, on at least one occasion, was accompanied by Said Chedadi, another member of the Madrid cell involved in fundraising for the "Chechen brothers." Chedadi is known to have been close to Mohammed Chaoui, one of the three Moroccan men arrested on March 13 by Spanish authorities for their involvement in the deadly Madrid bombings. Another Moroccan alleged to be involved in the bombings, Jamal Zougam, was found with several tapes about jihad in Chechnya, when Spanish authorities searched his Madrid apartment in July of 2001.

While the Chechens serve as the most likely model, there is no shortage of examples of planned and successful attacks on the world's rail systems. Prior to 9/11, terrorists bombed the Paris metro and released poison gas in the Tokyo subway system.  In addition, since 9/11, counterterrorism agents have broken up plots to launch a cyanide attack on the London Tube and bomb railway stations in Dresden and Madrid.

In the United States, the rail network has also been repeatedly targeted. On July 31, 1997, the NYPD launched a pre-dawn raid on an apartment in Brooklyn, New York, after receiving information that two men living in the apartment planned to bomb the New York City subway system. During the raid, police discovered nail-studded pipe bombs, one of which, in the words of a senior law enforcement official, was "all set and ready to go." NYPD Commissioner Howard Safir remarked, "these individuals intended to take these bombs onto subway trains, set them off, and the probability is that they and many others would have been killed."

The vulnerability of the New York City subway system again came into focus in September 2003, when Time magazine reported that Saudi Arabia had detained a terrorist with extensive knowledge of a plot to launch a poison gas attack on the subways.   In April 2003, news broke that another captured terrorist, Al-Qaeda operations head Khalid Sheik Mohammed, had informed interrogators of an Al-Qaeda plan to target Washington D.C.'s metro.

The warnings from Mohammed and the detainee in Saudi Arabia roughly corroborated an October 2002 FBI statement that "information from debriefings of Al-Qaeda detainees as of mid-October indicates that the group has considered directly targeting U.S. passenger trains, possibly using operatives who have a Western appearance." The statement also noted, "recently captured Al-Qaeda photographs of U.S. railroad engines, cars and crossings heighten the intelligence community's concern of this threat."

The information gleaned from the detainees, coupled with the foiled 1997 Brooklyn bombing plot, make clear the peril posed to the U.S. rail system. When this bleak picture is merged with the international threat assessment, it seems likely that the horrors of Madrid may be repeated in the not so distant future.

Josh Lefkowitz and Lorenzo Vidino are Senior Terrorism Analysts at the Investigative Project, a Washington DC-based counter-terrorism research institute.