In Washington, D.C., Metro police were conducting random inspections of stations and rail yards, officials said. Atlanta's public transit system said its police department was increasing the number of officers and patrols throughout the system.
Russian authorities said two women blew themselves up in Moscow on Monday in a subway jam-packed with rush-hour passengers, killing dozens. They blamed the carnage on rebels from the Caucasus region.
The federal government did not immediately make any recommendations for increased security at mass transit systems, but authorities were monitoring the situation, a U.S. official said.
The official spoke on condition of anonymity to discuss internal deliberations.
Caucasus Islamic separatists tend to be focused on targets in the region, primarily Russia, and are not generally considered a threat to U.S. domestic interests.
"The actual Chechen rebels generally don't care about the U.S. one way or the other," said Jeffrey Mankoff, an adjunct fellow for Russian Studies at the Council on Foreign Relations in New York. "They are mainly interested in what's happening in Russia."
Subways have been an attractive target for terrorists: a large number of people in a concentrated space and fairly limited security measures, he said. London and Madrid also have had transit system terror attacks. Last month, Colorado resident and Afghan immigrant Najibullah Zazi pleaded guilty as the leader of a plot to bomb the New York subway system.
"The next frontier of Homeland Security will be on how you can tighten up rail security like airline security is tightened," said Raymond Tanter, who teaches "Terrorism and Proliferation" at Georgetown University. He said volume is one of the biggest problems: the Moscow subway system carries about 7 million passengers on an average work day, making it difficult to examine each passenger.
Some U.S. cities took extra precautions in reaction to the Moscow bombing. Others were confident their existing security was sufficient.
In Chicago, the city police department's public transportation section and Chicago Transit Authority personnel were watching closely for any suspicious activity or behavior, said CTA spokeswoman Kim Myles. Representatives of transit agencies in Boston and Philadelphia said they believed their normal security practices were vigilant enough to protect the riding public.
The New York Police Department issued a statement saying it was increasing coverage of the city's subway system as a precaution "in response to the Moscow bombings."
The city's Metropolitan Transportation Authority acknowledged heightened security, but declined to provide details. The agency is in charge of New York City buses and subways, as well as suburban trains, and bridges and tunnels.
New York City "did ramp up our coverage a little bit this morning" after officials learned of the Moscow bombing, said Mayor Michael Bloomberg.
"We change it every day, and for security reasons obviously we're not going to tell anybody what we're doing," Bloomberg said. "But you can rest assured we have great interest in what goes on around the world."
In Manhattan, where the public has grown accustomed to increased security after the 2001 terror attacks, many people said they hadn't even noticed the added measures.
"I don't think it poses a threat here now," said Carlos Rivera, 44, of Newark, N.J., who commutes to New York City daily and works in sales.
"Every day, I see the NYPD out here. I see the dogs. I can't let it affect my life right now," said Rivera. "I don't think about terrorism. I only think about it when I hear about it. Other than that, it never enters my mind."
Andrew Davis, 24, who was catching a train home to Morristown, N.J., said he feels safe and didn't notice any increased security.
John Villegas, who said he used to work near the World Trade Center, did sense the heightened security.
"I'm a little wary," Villegas, 48, said at Pennsylvania Station as he waited for a train home to Woodbridge, N.J. "I do not feel safe right now. It's a little scary."