Homeland Security officials on Thursday were keeping close watch on developments related to the terrorist attacks that killed or wounded thousands of train riders in Spain.
The attacks have not prompted the United States to raise its terror alert, which is now at yellow, an elevated level. Homeland Security spokesman Brian Roehrkasse said that based on the current assessment of intelligence "we do not have similar corresponding threats directed to the U.S."
That could change as the investigation continues, he said.
Amtrak spokesman Dan Stessel said the railroad is continuing to review information received from the FBI and the Transportation Security Administration.
He said there's no credible threat against Amtrak or other railroads.
Nevertheless, the passenger railroad is taking extra precautions.
Amtrak increased patrols of its police force and canine units, Stessel said. Electronic surveillance of bridges and tunnels was intensified, he said. And the company reinforced its message to Amtrak employees to report suspicious activities to police.
"That gives you another 20,000 sets of eyes," Stessel said.
Acting TSA chief David Stone said the agency has been working with public transit systems to close security gaps.
"TSA is very much involved in all risk mitigation plans with trains, metropolitan transit systems and ports," Stone said on Thursday.
Bruce Hoffman, a terrorism expert at the Rand Corp., said it's much harder to secure transit systems than airports because it isn't possible to closely scrutinize every person in such large crowds.
"If terrorists want to kill a lot of people, public transportation is always the preferred target, because you get a lot of people in the same place at the same time," he said.
Though airplanes continue to be an attractive target for terrorists, he said, the U.S. government's tightening of airport security may have made public transit more vulnerable.
"You harden one target and you shift the threat to another," Hoffman said.
Further, he said, the success of the attacks on trains in Madrid may inspire other terrorists to imitate them.
If the TSA gets wind of a threat, the agency's communication system allows it to communicate quickly with transit agencies, railroads, bus companies and cruise lines, Hatfield said. The agency has given $115 million in security grants to transit systems in the past year, he said.
James Carafano, a homeland security expert with the conservative think tank Heritage Foundation, said al-Qaida has shown an interest in bridges and tunnels. Major subway systems are now closely monitoring tunnels, he said.
"The major subway systems are better prepared than they were several years ago, and that's encouraging," Carafano said.
The Federal Transit Administration has sent technical teams to transit systems to assess their vulnerability and given them grants for training their employees.
In Washington, metro subway platforms were cleared of fixtures that officials thought could be hiding places for bombs. Officials removed trash cans, newspaper recycling bins and newspaper sales boxes.
Mailboxes disappeared from downtown Washington streets as the city tightened its guard against terrorism.
Last month, Homeland Security officials met with a Russian delegation for a debriefing on the February explosion on the Moscow metro that killed 41, Hatfield said.
"You can bet we're going to be talking to our allies in Spain and try to boil down lessons learned as the investigation into this horrible attack take place," he said.