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The aborted shooting attack on a European high-speed train provides a sobering reminder that no other form of transportation has the kind of security air travel does — a hard fact unlikely to change.

"The challenge of securing mass transit is one that will persist," says David Heyman, a former top official at the Department of Homeland Security. "You have a system that is open to the public and requires rapid turnaround of people in subways and rail."

Belgium is increasing baggage checks and uniformed patrols on its high-speed trains as a result of Friday's attack on an express train from Amsterdam to Paris. Passengers, including three Americans, subdued the attacker.

A spokesman for the Department of Homeland Security said Monday no new warnings were issued in the U.S. as a result of the incident and that no new threats had been detected.

Related: Hero Americans Who Thwarted Train Attack Awarded Legion d'Honneur

The vulnerability of rail travel was vividly exposed in 2004 when ten bombs ripped through Madrid commuter trains, killing 191 people and injuring than than 1,800. A year later, suicide bombers killed 52 people in explosions on board the London Underground and a double-decker bus.

In the United States, transportation security has concentrated most heavily on air travel as a result of the 9/11 attacks and continued attempts to bring down passenger jets. The federal government spends roughly nine times as much on aviation security as it does on securing rail travel.

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Uniformed officers with bomb-sniffing dogs routine patrol train stations in big cities. Passenger IDs and baggage are randomly checked. The Transportation Security Administration also deploys special teams that bolster security at train, subway, and bus stations.

But some members of Congress have been critical of the concentration on air travel.

"Anybody who's looking at a map of terrorist attacks since 9/11 would say, 'Hey, wait a minute, rail is really important," said Sen. Corey Booker, a New Jersey Democrat, at a budget hearing last spring. "There have been hundreds and hundreds and hundreds of deaths due to terrorist attack on rail — on rail, on rail, on rail.”

An analysis by Prof. Arnold Barnett, an MIT statistician, found that of 32 acts of terrorism against transportation systems between 2002 and 2011, only 9.4% were directed at aviation.

Barnett's study, published in the journal "Risk Analysis," found that 206 air passengers were killed in terror attacks during that period, compared to 425 on long distance trains and 906 on subways and commuter lines.

Applying airport security to train stations, however, would be astronomically expensive. Most major railway stations were designed around 100 years ago to quickly move a large number of people on and off trains, and the passenger volume is huge.

Figures provided by Chicago's busy O'Hare International Airport show it handles roughly 70 million passengers a year, a figure dwarfed by the roughly 273 million people who pass through New York's Grand Central train station each year.

Those figures, and the way train tickets are sold, would make it very difficult to apply the government's terror watch lists to train travel.

Related: American Professor Mark Moogalian 'Rushed' Train Gunman, Was Shot

European authorities have said that the gunman in Friday's attack was known to intelligence services as a suspected radical.

In the U.S., a person on a watch or "no-fly" list would not be flagged buying a train or bus ticket. Sen. Charles Schumer, a New York Democrat, called for creating a "no-ride" list for Amtrak in 2011, a proposal the rail line said "could add value in creating an overall security posture."

But current and former federal officials say such an arrangement would require a change in federal law and a total overhaul of the system for selling train tickets, which allows passengers to buy tickets just minutes before a train departs.

Heyman, the former DHS official, said last Friday's attack is reminder of the effectiveness of "collective security."

"Passengers, not security experts, are often the ones who solve these lone wolf problems," Heyman said.