The New York, New Haven and Hartford Railroad train derailed at a grade crossing, and seconds later a Chessie System freight highballing out of a tunnel struck a stalled passenger train. A railroad company's worst nightmare? Not this time, because the trains are models at the B&O Railroad Station Museum, and the disasters, said museum director Jack Mitchell, are part of "the fun of it."

It's Christmas garden season at the Ellicott City museum, where there's no trouble finding fun. Here the railroad era lives in miniature, through the model trains that chug past tiny townscapes beneath Christmas trees in every room of the two-story museum.

Here children (and adults) can don their striped caps and play engineer. In some exhibits, visitors can toot whistles and maneuver vintage Lionel trains (among other models) around tracks. At others, visitors can watch the sets come alive as electric scenery pieces swing into motion: Toy bulldozers plow sand, oil derricks pump away and swans gambol about in a pond.

At this time every year, Mitchell and his volunteer staff remove hundreds of model train cars from the display cases and put them on the tracks, so that engines can haul cars filled with tiny circus animals and lumps of coal through miniature villages. It is a holiday treat that makes children and plenty of adults light up with smiles.

Christmas garden is the name that generations of Baltimore residents have given to the tradition of placing model train sets around the base of a Christmas tree and decorating the area with miniature town scenes. City folklorist Elaine Eff said the garden tradition has its roots in 19th century German communities in Pennsylvania.

Although the tradition's popularity waned in the 1960s, Eff said it is back in full force because of the resurgence of interest in Americana and trains.

At the museum, the interest is high. Mitchell, showing a visitor around the exhibits, stood over the museum's most complex train set trying to get the kinks out of the system. The trains were running at full speed in every direction, whistles blowing, bells clanging and lights flashing.

To the inexperienced visitor, the operation appeared to be running smoothly, but Mitchell was waiting for a kink to pop up. Sure enough, a rear car on the Alton Limited toppled off the track. Chuckling, Mitchell said, "You'd never run a real railroad this way."

The youngsters don't mind, however. They squeal and shriek every time a train passes. Matthew Temption, 3, and his sister Erica, 18 months, from Glenelg, had the controls to themselves one afternoon recently and gleefully ran the trains and sounded whistles.

"This is fun," said Matthew, his eyes fixed on the train circling in front of him. When it was time to go and his baby-sitter tried to steer him away, he said, "Just one more time."

It takes a year's worth of work to pull off a show of this size. The cars and tracks have to be maintained the same way a real railroad does.

"All summer we tear up tracks and clean and lubricate the cars. And there are always engine repairs," Mitchell said. "During the busy season, engines have to be lubed every 50 hours. When you're running them over 20 hours on the weekends, it's a lot of cleaning."

The museum, filled with artifacts and photographs, focuses on the history of the Baltimore and Ohio Railroad, the company that built the old Ellicott City depot that houses the museum. At Christmas time, some of the museum's regular exhibits are cleared away, and the dozen or so model trains, some with locomotives as "small as your little finger and others as big as bread boxes," roll in, Mitchell said.

The collection of model trains runs from the steam era, when engines looked like "boxes on wheels," Mitchell explained, "through the sleek, streamlined diesel and electric styles of the 1930s to the present locomotive designs you see on the railroads today." There are some reproductions, but many of the trains are the ones actually built during those periods by the legendary model train companies, Ives, Lionel and American Flyer.

Model train watching is not for children only. The six-week event draws about 4,000 people to the museum each holiday season, and many youngsters arrive with grandparents in tow.

"These were the engineers on living room railroads" of Christmases past, Mitchell said, "who come to the museum to remember their childhoods. Then the little kids who stream in behind them and want to see what their grandparents' childhood was like."

The B&O Railroad Museum is at Maryland Avenue and Main Street in Ellicott City. The regular hours are 11 a.m. to 5 p.m. Wednesday through Saturday and noon to 5 p.m. Sunday. Through Jan. 31, the museum will have extended hours, which will vary. There is an admission charge. Call (301) 461-1944 for more information.