Speculation is rife as to who was responsible for fixing six metal plates on a stretch of railway in Germany over the weekend as Europe's rail networks face up to the possibility that thousands of kilometers of untended track could be open to terrorist attack.

The plates, measuring 3 centimeters thick and more than 30 centimeters long, were bolted to the rails by unknown assailants near the western city of Dortmund on the high-speed Cologne to Berlin Inter City Express (ICE) line.

The driver of an approaching ICE train carrying 200 passengers spotted the plates, which were covered by blue garbage bags, and was able to brake sharply, slowing the train to about 90 kph (55 mph), before hitting the slabs, police said on Sunday. ICE trains passing the point where the slabs were bolted usually reach 160 kph on the track there. Although the train ran over the metal plates, it stayed on the rails and no one was injured.

Police suspect group involvement

Police in the state of North-Rhine Westfalen said that it was unclear whether the incident was the work of terrorists, but they speculated that the attempt to derail the train would have required more than one person as the plates weighed 17.5 kilograms each and were fixed on a stretch of track exposed to heavy traffic.

"At this point we don't believe that this was a terrorist attack," said state prosecutor Henner Kruse at a news conference in Dortmund. Kruse added that they had no clues on suspects or the motives as yet.

Despite the lack of evidence at this time, an incident which appears to be a concerted effort to derail a high-speed train immediately prompts thoughts of the possible targeting of European railways by terrorist organizations. Speculations of such a campaign gain additional credibility after the discovery of another bomb beside a train track in Spain, just three weeks after the devastating March 11 attacks on the Madrid rail network which killed 191 people and injured 1800.

Europe raises alert after Madrid

In the wake of the Madrid bombings, French authorities put train stations on a red alert, the second-highest of its four levels of emergency preparedness, after the discovery of explosives on tracks near the town of Troyes, 200 kilometers (120 miles) east of Paris, and another device under rails in central France last month. The French government assigned 500 additional police officers and 250 extra soldiers to patrol train stations, commuter rail stations and airports in Paris as a result. A group calling themselves the AZF claimed responsibility for the discovered bombs.

If the rail network becomes the next transport system to be targeted then European interior ministries face a massive task in trying to protect railways that stretch out from populous cities to the most remote corners of a continent that covers over 10 million square kilometers. Germany alone has a 40,000-kilometer rail network while the SNCF, the French national railway, operates between 12,000 and 14,000 trains a day, carrying up to three million passengers.

Holes in network security

With extra police officers and soldiers patrolling stations from London to Rome, network hubs have seen a marked increase in security but the discovery of explosive devices and attempts at life threatening disruptions to trains outside of major conurbations suggest that terror goals can be achieved away from the more obvious targets.

The threat is not exclusive to Europe's rail networks. In the United States, Federal counterterrorism officials issued a bulletin to police departments across the country last week warning that terrorists might try to bomb rail lines and buses this summer.

Monitoring a huge task across Europe

"How can you secure these environments?" said Rolf Tophoven, a prominent German terrorism expert in a recent interview with the New York Times. "You can bring in more police. But you can't X-ray every single piece of luggage. It's impossible to put up a total security net."

When a tractor can roll onto a high-speed line and derail an Intercity train without anyone knowing until it's too late, as it did only last week in Germany, placing a bomb on a secluded rail track should present few problems. The problem for European security services facing a possible rail bombing campaign is how to monitor and control the continent's vast, unsecured networks.