Tiny screwdriver in hand, Steven Clarke probes the innards of a stricken transformer, the main throttle for the Brandywine River Museum Model Railroad.

"Never trust inanimate objects," he declares. "Yeats was right: Things fall apart. Whatever can go wrong will go wrong, usually at the most inopportune moment. But the show must go on."

This year, for the 32d time, Clarke is in charge of the show, which is the star attraction of the museum's Christmas celebration. During the rest of the year, paintings and other artifacts of Wyeth worship draw visitors to this esteemed Chadds Ford cultural repository, but from Thanksgiving to early January, high art yields to the common and enduring appeal of a magnificent miniature railroad.

"People come here to see the trains," Clarke says. "It won't do to say they're not running."

On this day, with the help of a substitute transformer purchased at a flea market, life was proceeding with the usual animated magic in the Lilliputian towns and villages overseen by Clarke. Five trains were running - including a freight train with 70 cars - and whatever mechanical problems were furrowing Clarke's brow were completely invisible to a shifting crowd of spectators, heavily skewed in age toward single digits, whose responses to his handiwork ranged from awe to delight.

How could it be otherwise? The E-shaped layout is immense - 35 by 61 feet - and traversed by about 2,000 feet of 0-gauge track. Besides seemingly infinite freight trains, there are passenger trains sporting the colors of such storied lines as the Santa Fe, Union Pacific and Pennsy, as well as a streamlined California Zephyr.

Clarke and associates such as engineer Dave Jensen (who is licensed to operate real locomotives) have created an enchanted and enchanting landscape.

The wintry countryside is enlivened by features that display not only imagination (intelligence having fun), but something even more precious - a childlike imagination.

Next to a waterfall where real water flows, knights joust at a medieval festival. Skaters etch circles on a pond. Imitation smoke curls from the stack of the power plant next to the roundhouse.

At the drive-in theater, the Christmas classic Rudolph the Red-Nosed Reindeer plays on the outdoor screen. There's a replica of the Herr's snack factory (an exhibit sponsor) and a Berks Products stone quarry and processing plant (another sponsor).

While the exhibit's ingenuity will impress model railroaders, it is designed primarily to entertain families, not hard-core rail fans, who obsess over detail and authenticity. In the switchyard, Thomas the Tank Engine and his brightly colored locomotive friends from the Island of Sodor, all hugely popular, wait for the next chance to amuse the under-5 crowd, a task at which they rarely fail. Over part of the display, Santa and his reindeer fly.

New this year is a salute to "the men behind the trains" - short biographies and photographs of such toy-train pioneers as Lionel Cohen, A.C. Gilbert, Harry Ives, Louis Marx and John Tyler, whose Mantua Metal Products company, once headquartered in Woodbury Heights, N.J., spurred the development of HO-gauge trains.

Clarke, 60, of West Chester, is officially the curator of the exhibit. On his business card, he calls himself a toy trainsmith. Neither title fully captures the man. He is steeped in history, pregnant with knowledge, Edisonian in inventiveness, and resolutely eccentric.

With his sharp features, tousled hair, wire-rim spectacles and prominent chin whiskers, he resembles an Amish farmer. On this day, he is wearing a dark-blue chamois-cloth cavalry shirt under an Army-surplus tool vest. His denim jeans are cinched with a tan Boy Scout belt that symbolizes his curatorial philosophy: Be prepared. To that end, he carries in his vest a conductor's pocket watch on a chain, a small flashlight and a plastic tube for blowing out clogged locomotive smokestacks.

His face bears the expression of a man beset by a hundred petty vexations, all of which he intends to vanquish by day's end.

While Clarke cheerfully answers questions and dispenses lore, he is mindful of short attention spans and tries not to overwhelm. He favors puckish epigrams to windy bombast, showing rather than telling. Children are mesmerized by him; they know instinctively that they're in the presence of a character, a subversive, a kindred spirit who sees and thinks the way they do.

Toy trains today have become electronic marvels, with all sorts of digital bells and whistles. Clarke eschews such computerized gimcrackery, preferring to stay simple, old-school and mechanical, mainly for practical reasons.

"If something goes wrong, it's something I can deal with, in the dark and in a crowd," Clarke says. "I can't say, 'The system is down. Please come back in a couple of weeks.' "

Setting up the display is a process that takes at least four grueling days and entails transporting 29 layout sections - veritable tectonic shards that are both heavy and fragile - from a barn in northern Delaware. In addition to all his other talents, Clarke has a strong back and an exquisite sense of space.

The rest of the year, Clarke fixes and spruces up the disassembled layout and toils on new projects and additions. He also does jobs for private clients, restoring and repairing old trains, designing and building layouts.

"I've seen a resurgence of interest in Christmastime trains," Clarke says. "It's something you can do at home that involves the whole family. And it's a lot less expensive than going to Aspen or Europe."

Some of the kids and young adults who came to see the trains 32 years ago when Clarke began tending the exhibit are now returning with their children and, in some cases, grandchildren.

"The thing I enjoy most is seeing a family sitting on a bench watching the trains, cuddling and hugging, while the parents and grandparents talk about their own childhood memories," Clarke says. "The trains are just a way to help people feel good."