TSA VIPR team members in Memphis, Tenn. TSA The next time you have to take off your shoes and get searched by an employee of the Transportation Security Administration, it may not be at the airport.

Ron Nixon reports in The New York Times on the expansion of TSA's Visible Intermodal Prevention and Response squads, or VIPR, which are leaving the airports behind to perform security checks at train stations, subway stops, and other transportation-related hubs.

Created in the wake of the 2004 Madrid train bombing, VIPR teams — comprised of explosives experts, behavioral detection officers, and canine handlers — work with local law enforcement to move through crowds and randomly stop passengers and ask questions.

Perhaps more surprising is their reach into places you wouldn't really expect: Rodeos, music festivals, and sporting events. Their expansion has civil liberties groups pushing back on what they call warrantless searches with no probable cause.

Writes Nixon:

“The problem with T.S.A. stopping and searching people in public places outside the airport is that there are no real legal standards, or probable cause,” said Khaliah Barnes, administrative law counsel at the Electronic Privacy Information Center in Washington. “It’s something that is easily abused because the reason that they are conducting the stops is shrouded in secrecy.”

T.S.A. officials respond that the random searches are “special needs” or “administrative searches” that are exempt from probable cause because they further the government’s need to prevent terrorist attacks.

According to the TSA's official website, "TSA routinely conducts thousands of VIPR operations each year in transportation systems nationwide" — nearly 4000 in 2010 according to CNN.

Officials told CNN that the searches are not mandatory — if your definition of "not mandatory" is not riding the train if you refuse a search at the train station.

"At the airport, everybody now understands it's part of the process," Rick Vetter told CNN. "You can either choose to deal with it or not. But in a surprise situation like that, I would not have been pleased."

Despite the concerns of civil libertarians, security experts heap praise on the teams.

“They introduce an unexpected element into situations where a terrorist might be planning an attack,” Rafi Ron, the former chief of security at Israel's Ben-Gurion airport, told The Times.