US air marshals are expanding their work to bus and train stations, ferries and other mass transit facilities under a three-day test programme.
The TSA is trying to expand the role of air marshals
Undercover marshals and uniformed officers will be conducting anti-terror surveillance, the Transportation Security Administration (TSA) said.
The TSA is trying to expand the role of air marshals to bolster home security and give officers more career paths.
The exact number of air marshals working across the US is classified.
Officers moved out of their usual role of flying undercover on aeroplanes after Hurricane Katrina struck New Orleans, keeping order at Louis Armstrong International Airport.
From Wednesday, "Visible Intermodal Protection and Response" teams - or Viper teams - will patrol Amtrak's North-east Corridor and Los Angeles rail lines; ferries in Washington state; bus stations in Houston; and mass transit systems in Atlanta, Philadelphia, Washington and Baltimore.
"TSA expects to find new ways to quickly deploy resources, in the event of an actual threat, that adds complexity to security measures outside of the aviation domain, the TSA said in a statement.
American Airlines pilot Denis Breslin, spokesman for the airline's pilots' union, said air marshals ought to stick to aeroplanes.
"I don't think there's enough air marshals to cover commercial aviation as it is. That's what transit police are for," he said.