Step inside the Snook barn in Frederick County and enjoy the beauty of old age. Century-old beams of white oak are like artwork, each with a different pattern of gouges left by hand axes. Roman numerals carved by 19th-century carpenters mark the posts holding up the roof. The sturdy pine floorboards, each nearly two feet wide and two inches thick, were carved from an ancient virgin forest that no longer exists.

Not so long ago this barn, part of the Snook family farm north of Frederick, which will be part of a new county park, was near collapse. Then, a little-known team from the National Park Service began working to make sure it will be around for a long time to come.

Since January, craftsmen from the agency's Historic Preservation Training Center in Frederick County have been shoring up the beams, reusing original materials and using age-old woodworking methods to restore the barn. They have repointed the stone foundation and spliced the beams together with styles of joints that were common before the use of iron nails became widespread.

But the crew is leaving behind more than a refurbished structure. Like all of the center's projects, the restoration has doubled as a hands-on classroom. While preserving buildings, the center's craftsmen also teach skills in masonry, carpentry and other trades to those who look after historic structures in local and federal parks.

Last week, for example, Greg Dodson, 53, a National Park Service employee who lives in Torrington, Wyo. (population 5,000), was crawling around the scaffolding inside the Snook barn to take what he learned about fixing historic wooden structures back to Fort Laramie, a storied outpost in the years of America's westward migration and Indian wars.

"What makes this a really interesting place are the people who work here," said H. Thomas McGrath Jr., 54, superintendent of the Historic Preservation Training Center. "They're passionate, they're enthusiastic and they're extremely competent. What's even better is they're willing to share their knowledge."

The Snook farm is one of nearly 700 historic preservation projects undertaken by the center in the past 15 years. In Hawaii, a team from the center has been restoring historic buildings at the Kalaupapa National Historical Park on the island of Molokai.

From 1866 to 1969 the north shore of the island, with its 3,000-foot cliffs, was turned into a compound where 8,000 people who had leprosy were forcibly isolated in what was considered a "living tomb." The team from Frederick was there last week helping to replace termite-damaged wood in 14 buildings dating as far back as the 1920s.

Their handiwork also can be seen at Harpers Ferry in West Virginia, Fort McHenry in Baltimore, the lighthouse at Cape Lookout in North Carolina and the log cabins where Gen. George Washington's troops sheltered at Valley Forge, Pa. They have rebuilt cannon carriages for President's Park at the White House and refitted windows in Independence Hall in Philadelphia.

Closer to home, the center has helped preserve the Gambrill House, a mansion built about 1872 at the Monocacy National Battlefield south of Frederick that now serves as the organization's administrative headquarters. In partnership with Frederick, the center has helped transform the former Jenkins Cannery in downtown Frederick into its workshop.

"What appeals to me more than anything is the fact that I get a chance to work on America's premiere structures," said Chris McGuigan, 45, an exhibit specialist with the training center. "I could probably give you 100 buildings that people would die to say they've worked on."

Established in November 1977, the center was an outgrowth of efforts to restore the C&O Canal. Having acquired hard-won skills in rebuilding locks, aqueducts, culverts and the houses of workers who tended the locks, the crews saw a continuing need for those skills in other federal parks. Today, the center has more than 50 employees, according to McGrath, who took over in 1990.

The center's crews work side by side with other workers on state and local projects as well as restorations by nonprofit groups such as the National Trust for Historic Preservation. Though it is a part of the National Park Service, the center acts as an in-house contractor and is paid for each job.

On each project, the craftsmen emphasize the importance of reusing original materials when possible. But the workers have room for creativity: They often go freestyle, finding solutions to architectural problems as they uncover them.

"One of our students has a master's degree from [the University of Pennsylvania], and we have him pushing a wheelbarrow around," McGrath said of a preservationist being instructed at the center. "Why's that? Because these people have a passion for the hands-on, for that process of discovery."

At the Snook farm project, the barn wasn't the only structure falling apart. Some of the outbuildings -- the hog barn, the carriage house and the corn crib -- seem to be dissolving into a sea of weeds. The brick farmhouse, built around 1840, appears to be in fair shape.

Known as a bank barn because the structure abuts an earthen bank, the German-style barn once housed livestock, a granary and a hay loft. Its structural integrity had been compromised by an owner who removed beams to install a mechanical tram that brought in bales of hay.

"This whole side of the building was falling off and bringing the rest of the structure with it," McGuigan said.

First, workers stabilized the structure with metal supports. Then they set about to rebuild it with wood.

Dean Wigfield, 45, the project manager at the Snook barn, said he believes that the barn was laid out and begun by a master journeyman. But then the degree of craftsmanship deteriorated: Scarf joints, which can be tricky to fashion but provide great stability, gave way to half-lap joints, which are easier to construct but potentially more wobbly.

Many of the center's workers say they have passed up more lucrative jobs in the private sector because they relish the idea of working on buildings with historic significance. They enjoy searching for clues about a building's construction and the people who built it. They admire the ingenuity of early builders and sometimes rue the disappearance of those skills in a world of prefab houses.

They share stories of those goose bump moments of finding a trove of yellowed records in some rafters or peeling back a piece of molding to find the date and signature left by a carpenter.

"All of a sudden you're in touch with this person," McGuigan said. "You're connected."

Crews from the Historic Preservation Training Center have been using age-old woodworking methods to restore the 1840s barn on the Snook farm north of Frederick. Below at left are wood chisels. Chris McGuigan, left, an exhibit specialist at the Historic Preservation Training Center, and Dean Wigfield, a project supervisor, discuss hand-forged nails used in the barn. At left are posts marked with Roman numerals. The center has been working on the barn for about nine months.Greg Dodson, left, a Park Service employee from Wyoming, and Chuck Anderson of the Historic Preservation Training Center in Frederick County work on the Snook barn. The Gambrill House, below, near Frederick, has been restored.