In this March 17, 2016, photo, a TSA K-9 handler walks his dog through lines of travelers approaching a security screening checkpoint at Seattle-Tacoma International Airport in Seattle. (AP Photo/Ted S. Warren) (Ted S. Warren/AP)

Although terrorists have attacked trains and subway systems in Europe, there is relatively little risk that they will assault similar targets in the United States, according to the nation’s transportation security chief.

“Right now we consider the general risk to be low in the United States domestically against surface transportation,” said Peter Neffenger, administrator of the Transportation Security Administration (TSA). “That’s freight rail, light rail, metro subway systems, over the road buses and the like.”

Hundreds of people have been killed or injured in Europe by terrorists who attacked trains and subways or, in one case in France, mowed down pedestrians with a car. The most recent assault was an explosion aboard a Brussels metro train last month that killed 16 people. The most deadly came in 2004, when 10 explosions on four commuter trains in Madrid killed 192 people an injured about 2,000.

Neffenger, in a conversation with reporters on Friday, said that is unlikely to happen in the United States.

“It doesn’t mean there’s no threat, there’s no risk, but it’s relatively low,” he said.

[Brussels airport bombings bring new security measures in U.S.]

Some of his assessment comes from “a vast national intelligence network” and is classified information, he said, but the rest evolves from common sense or, perhaps, seeing the situation through a terrorist’s eyes.

“We know that there is a huge psychological benefit from the terrorist’s perspective if you can attack something like the aviation system successfully, because we’ve put a lot of effort into protecting it,” he said.

Terrorists know that any form of attack will show their hand, likely leading to their arrest if they survive the attack.

“If you’re going to do something, you’re going to have to surface yourself in the process of doing so, so I think terror groups, and individuals that want to do harm, want to ensure that if they’re going to expose themselves, they get the maximum return on that exposure,” Neffenger said.

That means attacking an airport or airplane, he said, but he was quick to add that TSA works with transit and railroad police, as well as local law enforcement, to protect the non-aviation sector of the transportation system.

“I don’t know that there would be any way to provide aviation-style security in the public transportation world without changing the entire way we get around,” he said. “Fortunately, we’re not seeing credible intelligence threats that tell us that that’s an issue for the United States.”