Greg Hull, security chief for the American Public Transportation Association, said Friday the transit systems are at "code yellow-plus" following the bulletin about a possible terror plot from the FBI and the Homeland Security Department.
CBS News Correspondent Stephanie Lambidakis reports the warning urged rail and bus industry personnel to step up security and passenger screening.
U.S. officials said they had received uncorroborated intelligence reports about a plot by terrorists to target commercial transportation systems but had no information about specific cities or dates.
A senior federal law enforcement official, speaking Friday on condition of anonymity, said the intelligence, coupled with the deadly commuter train bombings in Madrid, in which bombs went off inside backpacks, has increased the level of wariness about a similar attack in the United States.
"It should not be considered unusual that the FBI should issue this kind of a bulletin in the wake of what occurred in Madrid last month," the Amtrak passenger railroad said in a statement.
Homeland Security spokesman Brian Roehrkasse said information in the bulletin was being shared via the National Law Enforcement Telecommunications System to ensure proper security measures are implemented nationwide.
Officials said the message was sent mainly out of an abundance of caution, and the threat - deemed "somewhat credible" by one official - was not causing undue alarm throughout the government.
The nation's terror alert level remains at yellow, or elevated, the midpoint of the five-color scale. It was last raised to orange, or high, on Dec. 21 amid suspicions about terror attacks using commercial aircraft. The level returned to yellow on Jan. 10.
Passengers could see changes because of the bulletin. Federal officials are encouraging local transit authorities to conduct random passenger inspections and security sweeps of stations and to increase public announcements encouraging people to report unattended baggage or suspicious behavior.
Intelligence indicates a plot might involve bombs made of ammonium nitrate fertilizer and diesel fuel, similar to the explosive concealed in a rental truck that blew up the Oklahoma City federal building in 1995. Both items are readily available.
The improvised bombs would be concealed in luggage and carry-on bags, such as backpacks or duffel bags, and detonated either aboard buses or trains or in transportation stations, the government warning says.
on suspicion of being involved in a possible terrorist plot that included the discovery of 1,000 pounds of ammonium nitrate.
And police in Thailand say suspected Islamic rebels.
In addition to Oklahoma City, ammonium nitrate was used in the 2002 blast in Bali, at the World Trade Center in 1993 and in Pakistan in mid-March of this year. In years past, the Irish Republican Army used ammonium nitrate in attacks in London and in Northern Ireland.
Al Qaeda and other terrorist groups have "demonstrated the intent and capability" of attacking public transportation systems using a variety of bombs, the bulletin says. Attacks in Israel, Greece, Turkey, Spain and elsewhere have used suicide bombers or triggered bombs with timers and cell phones.
Between 1997 and 2000, more than 195 terror attacks occurred on transit systems worldwide, congressional investigators say.
On Friday,connected to a detonator with a 450-foot cable under tracks of a high-speed railway between Madrid and Seville.
In France, authorities last month found a bomb half-buried on a train track some 100 miles southeast of Paris andin connection with an investigation into a mysterious group's threats to bomb French railways.
In Boston, which is preparing for the Democratic National Convention in July, commuters have gotten used to increased security on public transit.
So news that security could be tightened even more in response to the new warning didn't faze most residents, who accepted it as a necessity after the 2001 terrorist attacks.
"If it can help make us a whole lot safer, the inconvenience is worth it," said Queen Owens, a 44-year-old Boston medical assistant. "After Sept. 11, I think we should all be worried a little bit. What's to prevent a surprise from really happening again?"
The warning was taken in stride in other cities as well.
Gabriel Banda, 19, of Dallas, who rides the bus every day, said he would not vary his plans because "What are the chances of that happening here?"
He was skeptical of any increases in security that include baggage screening. "Almost everyone in DART is carrying a bag. To search all those bags on one ride, it just would annoy everybody."
Massachusetts Gov. Mitt Romney asked the state public safety secretary to work with transit authorities to develop "more robust plans" to protect citizens on public transportation. He said officials would consider a number of options, including baggage screening and limiting access to parking facilities.
Dallas-based Greyhound Lines Inc. said Friday it already had a plan in place to deal with security for each terror threat level.
However, when a bulletin is issued, like the one given to law enforcement agencies Thursday night, company officials tell employees to be extra vigilant, said company spokeswoman Lynn Brown.
Dallas Area Rapid Transit Police Chief Juan Rodriguez said the 193-officer force has taken no extra precautions because it has been on a higher level of alert since the 2001 attacks.
In Chicago, a spokesman for the Metro commuter rail service said security was increased after Sept. 11 and last month's train bombings in Madrid, and that those security measures remained in place.
Spokesman Tom Miller said it would be difficult to secure everything all the time.
"The question is asked, 'Why can't you seal off train stations like airports?' We have a completely open system, with over 200 stations in our system. They are impossible to seal off like airports," he said.
More than 9 billion trips are taken each year on the U.S. public transportation system, with 32 million trips every weekday - about 16 times the number of trips taken on airlines, according to the American Public Transportation Association.
The association estimates that $6 billion is needed to upgrade and modernize U.S. transit systems to meet security needs. The Transportation Security Administration dedicated only $10 million for passenger rail and public transit security in the current year's budget, according to the House Homeland Security Committee.
"Failure to invest in the security of passenger rail and public transit could leave these critical systems vulnerable to terrorist attack," the committee's Democrats said in a letter to Homeland Security Secretary Tom Ridge. "Making these systems as safe as they can be from terrorist attack must be a high priority."
After the Madrid bombings, the Homeland Security Department announced a series of security initiatives, but with no major new funding plans.