Rick Vetter was rushing to board the Amtrak train in Charlotte on a recent Sunday afternoon when a canine officer suddenly blocked the way.

Three federal air marshals in bulletproof vests and two officers trained to spot suspicious behavior watched closely as Seiko, a German shepherd, nosed Vetter's trousers for chemical traces of a bomb. Radiation detectors carried by the marshals scanned the 57-year-old lawyer for concealed nuclear materials.

When Seiko indicated a scent, his handler, Julian Swaringen, asked Vetter whether he had pets at home in Garner. Two mutts, Vetter replied. "You can go ahead," Swaringen said.

The Transportation Security Administration isn't just in airports anymore. TSA teams are increasingly conducting searches and screenings at train stations, subways, ferry terminals and other mass transit sites around the country.

"We are not the Airport Security Administration," said Ray Dineen, the air marshal in charge of the TSA office in Charlotte. "We take that transportation part seriously."

The TSA's 25 "viper" teams - for Visible Intermodal Prevention and Response - have run more than 9,300 unannounced checkpoints and other search operations in the past year. Department of Homeland Security officials have asked Congress for funding to add 12 more teams next year.

According to budget documents, the department spent $110 million in fiscal 2011 for "surface transportation security," including the TSA's viper program, and is asking for an additional $24 million next year. That compares with more than $5 billion for aviation security.

TSA officials say they have no proof that the roving viper teams have foiled any terrorist plots or thwarted any major threat to public safety. But they argue that the random nature of the searches and the presence of armed officers serve as a deterrent and bolster public confidence.

"We have to keep them (terrorists) on edge," said Frank Cilluffo, director of the Homeland Security Policy Institute at George Washington University in Washington. "We're not going to have a permanent presence everywhere."

Subways, trains at risk

U.S. officials note that digital files recovered from Osama bin Laden's compound in Pakistan after he was killed by Navy SEALs in May included evidence that the al-Qaida leader had considered an attack on U.S. railways in February 2010.

But critics say that without a clear threat, the TSA checkpoints are merely political theater. Privacy advocates worry that the agency is stretching legal limits on the government's right to search U.S. citizens without probable cause - and with no proof that the scattershot checkpoints help prevent attacks.

"It's a great way to make the public think you are doing something," said Fred H. Cate, a professor at the Indiana University Maurer School of Law, who writes on privacy and security. "It's a little like saying, 'If we start throwing things up in the air, will they hit terrorists?' "

The TSA's viper program is increasingly active.

In Tennessee in October, a viper team used radiation monitors and explosive-trace detectors to help state police inspect trucks at highway weigh stations throughout the state. Last month in Orlando, Fla., a team set up metal detectors at a Greyhound bus station and tested passengers' bags for explosive residue.

In the Carolinas this year, TSA teams have checked people at the gangplanks of cruise ships, the entrance to NASCAR races, and at ferry terminals taking tourists to the Outer Banks.

Searching in Charlotte

At the Charlotte train station on Dec. 11, Seiko, the bomb-sniffing dog, snuffled down a line of about 100 passengers waiting to board an eastbound train. Many were heading home after watching the Carolina Panthers blow a 16-point lead and lose to the Atlanta Falcons.

No one seemed especially perturbed by the TSA team.

"It's probably overkill," said Karen Stone, 26, after a behavior-detection officer asked her about the Panthers game and her trip home to Raleigh.

"It's cool," said Marcus Baldwin, 21, who was heading home to Mebane, near Burlington, where he waits tables to help pay for computer technology classes. "They're doing what our tax money is paying them to do."