How safe are U.S. trains from potential attacks?

There was a clear show of force in the New York subway Monday: a heavily armed NYPD Hercules team swept through Grand Central Station as other officers conducted random bag checks.

"I've never had that happen, only at the airport," said Andrew Nielsen, who had his luggage searched. "I think safety is important. I don't mind sacrificing sometime for safety."

The Department of Homeland Security would not comment on security efforts following Friday's thwarted attack in France but it consists of undercover units and visible, uniformed police, including a TSA K-9 unit at Chicago's Union Station.

But rail travelers almost never go through screenings like at airports.

"Unfortunately here in America, we are a big soft target in many, many respects," said Ron Hosko, a former assistant director of the FBI.

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Passengers in New York City undergo a bag search at Penn Station. CBS News

He said that post-9/11, the focus was large-scale rail attacks like those in Madrid in 2004 and London in 2005. But the rise of lone wolf, ISIS-inspired attacks has law enforcement reassessing the threat.

"There is very unlikely to be security, somebody from law enforcement on every train, which presents an opportunity for a determined attacker," said Hosko. "And because this is essentially a container, it represents a likelihood of many casualties."

Law enforcement sources say there are no new credible threats against railways in the United States. But it remains a huge effort to secure. In just one example, Penn Station in New York City, more than half a million people pass through each day.