Police in New York are visibly increasing their presence after Tuesday's deadly bombings at Brussels' airport and metro system, authorities told NBC News.
The New York Police Department told NBC it was increasing security at mass transit points, bridges and tunnels and major city landmarks.
That has been a standard NYPD response in past to major terror incidents overseas.
Transit authorities in Washington also indicated, via tweet, that they would step up precautionary patrols Tuesday morning. Extra officers have been deployed and are on the look out for suspicious activity, an Amtrak spokeswoman told CNBC.
Frances Townsend, former Homeland Security adviser to President George W. Bush, said it is much more difficult to enforce spot checks in an open transportation system, such as a metro train or subway, than in a closed system, like an airport.
"The challenge there is you increase patrols and sort of random security measures to try and throw off any terrorist who is doing surveillance and trying to launch an attack, but [that is] very, very hard in open systems," she told CNBC's "Squawk Box."
Officials have beefed up security lines at airports because they believe the airlines are the targets, but those measures need to be more broadly applied, said Robert Liscouski, former assistant secretary for infrastructure protection at the Department of Homeland Security.
Rail and transit systems in particular have yet to receive the level of attention authorities have given airports, Liscouski said.
"As we keep pushing those perimeters out, we need to take equally as strong measures. They're going to have an impact on people's transit capability, but we do need to continue to push those perimeters out. But you're never going to be 100 percent safe or secure," he told "Squawk Box."
One strategy is to make it difficult for anyone to carry out a terrorist attack in an area where that attack could inflict considerable damage, Liscouski said. That would limit attackers to areas where damage would be limited, he said.
Increased security within airport departure areas may become necessary, Michael Chertoff, executive chairman of the Chertoff Group and former Department of Homeland Security secretary, told CNBC.
"I think the lines will get a little bit slower," said Tom Blank, executive vice president at Gephardt Government Affairs and former head of TSA Security Policy.
"The most important thing is to make sure that our European partners are beefing up their travel documents," Blank told "Power Lunch." "Are they checking and issuing passports legitimately? Are visas being issued legitimately? Are they looking for visa overstays, people who ignore and overstay the time they are allotted? And finding these kinds of indicators of bad behavior that will help us reduce the risk of the situation that unfortunately occurred today."
— CNBC's Anita Balakrishnan and Lenore Fedow contributed to this report.