The federal government will start screening checked bags next week on five trains leaving from Union Station, the first time this airport-style security measure will be used to safeguard trains.

Officials at the Transportation Security Administration said the experiment will last a month and is designed to find out whether screening checked bags is a useful way to protect against terrorist attacks on trains.

TSA officials said bags would be screened by the same three-dimensional imaging machines that have been used at airports since the Sept. 11, 2001, terrorist attacks.

The screening will occur from 9 a.m. to 5:30 p.m. weekdays and will affect only the five train lines that begin at Union Station and allow checked bags: the Carolinian, Silver Star, Capital Limited, Cardinal and Palmetto.

Safety on the nation's trains has received heightened attention since March 11, when a terrorist attack on a commuter train in Madrid killed nearly 200 people and injured nearly 2,000 others. TSA officials concede there is no way to safeguard every train and transit line in the country but said the measures being tested could bolster security at specific events, such as the Super Bowl, that might be attractive targets.

"What you can do is identify practices that work and either put them in place for a period of time, in a targeted geography or at a specific event," said TSA spokesman Mark Hatfield.

Train security is "obviously an area that we've been doing threat analysis" on, Hatfield said. "Checked bags are another area that pose a potential risk, and we look at this as a risk-mitigation measure."

The experiment is the second of three tests conducted by the TSA to bring airport-style security measures to the nation's rail and transit lines. But while airport screening focuses on weapons and metal objects that can be used as weapons, the train experiments screen people and bags for explosives that are a much greater concern on rail lines.

Last month, the agency used a high-tech "sniffer" on passengers at the New Carrollton Amtrak/MARC station to check them for bomb residue. Passengers had to pause in the "sniffer" for 12 seconds while a sensor in its ceiling looked for trace particles of explosives. Eight puffs of air aimed at the passenger's upper thighs shot out from vents in the equipment to help free any particles that clung to clothes. The air was powerful enough to flutter men's ties and suit coats and ripple women's blouses.

Also, passenger bags were placed on a conveyor belt and run through an X-ray machine.

TSA officials said initial results from that experiment showed that the machines worked well. They said they would have a full report in the coming weeks. There are no plans to install the machines permanently.

Next month, TSA officials said, they plan to screen passengers as they board trains. Passengers would have to go through a mobile checkpoint set up in the train's first or last car before proceeding to their seats. TSA officials said they haven't determined in what city or along what train line that test will take place.

TSA officials said they expect that, unlike those experiments, the screening of checked bags will have almost no impact on riders. Many, if not most, train passengers do not check bags, and those who do won't have to do anything differently, because their bags will be screened after they hand them over to Amtrak agents, TSA officials said.

Amtrak spokesman Dan Stessel said: "We remain committed to assisting the TSA in their efforts to identify best security practices for use in the rail environment. During the next phase of their pilot program, Amtrak will provide access to Union Station, logistical support and police assistance."

Scott Leonard, assistant director of the National Association of Railroad Passengers, said his organization has been supportive of the experiments and doesn't expect any problems with screening checked bags.

"It sounds like the test will be completely transparent to people who are checking bags," Leonard said.