The United States Department of Homeland Security reacted to today's bombings in London by raising the terrorist threat warning level at mass transit systems in major cities across the U.S.
In Washington D.C., as elsewhere, surveillance and security sweeps were stepped as commuters and other travelers continued on with their travels.
But, as VOA's George Dwyer reports, they did so with a heightened awareness of the potential threat.
"At the direction of the President we are working with the Department of Transportation, our other federal partners, state and local officials, and other transportation authorities to take all necessary precautions and to increase the security of our transportation systems and the citizens who ride them."
Travelers in Washington say they are aware of the increased security, and, for the most part, feel confident in using mass transit.
"I feel adequately protected. That's not something that's going to stop me from riding,” said one commuter.
"And we're just trying to be a little more observant and trying to watch out for any strange activity and other than that we're going about our own business as usual," said another traveler.
This couple from the Netherlands said they are wary, but not worried. "I heard that security is up this morning for this metro, but I'm not really scared."
His traveling partner added, "You are extra alert, you look around you. 'Do I see something that's not normal?' But also I think the security is extra high and they have brought heavy guards and they are taking care of us."
Raymond Tanter, a professor of security studies at Georgetown University in Washington, says passengers may have more to worry about than they know.
"There are millions of possible targets in the United States, maybe 500,000 or 600,000 in the United Kingdom. So public transport is one way of bringing down an industrialized democracy that depends on inter-dependence, communication, travel and the like."
AMTRAK, America's national inter-city rail system, is putting security officers on all trains, and asking its employees to be on alert for potential threats.