The reverse of the card directed readers to safety-related Web sites for further information.
Other transit systems posted statements on their Web sites calling for vigilance while reassuring riders that security measures were being implemented. For example, Faye Moore, the general manager of the Southeastern Pennsylvania Transportation Authority in Philadelphia, posted a statement saying, "I rode the train this morning, and I will ride it home at the end of the day. I believe the system is safe."
"We ask our passengers to be vigilant for suspicious objects or people," the statement said.
Electronic surveillance is also critical, said Mr. Livingstone. He noted that there are thousands of surveillance cameras in the United Kingdom in transit systems and elsewhere. U.S. transit systems need more cameras, he said.
Mr. Chase called for "deploying armed, sworn officers in front of these stations, ideally with dogs that are trained to smell explosive residue."
"Nothing beats a well-trained police officer, hopefully with a dog, pulling people aside and seeing if their sixth-sense suspicion" is correct, he said.
"We don't have nearly enough bomb sniffing dogs," said Mr. Livingstone.
"If somebody appears suspicious, they've got to grab them," Mr. Chase said.
More-drastic measures may be required to meet security needs, said Mr. Livingstone. "Ultimately, what you may have to do" is limit what people can carry onto mass transit. This might mean banning backpacks, he said. Doing so would "make it that much harder to leave something under the seats," he said.
"The problem is we have maybe 1/100th of the resources that we need," said Mr. Chase.
"But we have to keep in mind that there is no silver bullet to prevent these types of events from happening," said Mr. Grniet. "All we can do is take prudent mitigation measures and to remain vigilant and ask the public to remain vigilant."