Had his parents been able to afford one of those new fangled plastic train sets for his second birthday, John DeSantis might never have fallen in love with model trains. Instead, they bought a 1937 standard gauge Lionel locomotive from a neighbor, fueling their son's lifelong obsession with antique toy trains.

Fifty years later, Mr. DeSantis is still at it. Today, his private collection of vintage American Flyer, Dorfan and Lionel freight and passenger cars, trolleys and locomotives numbers in the thousands, filling more than a mile of shelving in the former Holmes Hall, the Renaissance Revival-style mansion he restored in the North Side's Allegheny West neighborhood. Culled from antiques shops, auctions and other collectors, it also includes an amazing assemblage of tin and cast-iron trains made by Ives Manufacturing Co., the largest maker of toy trains in the United States from 1910 until 1924. He has one of everything Ives ever made before the company went bankrupt in 1928.

"And everything is in original condition," notes Mr. DeSantis, executive director of the Pittsburgh Home and Garden Show.

For years, invited guests and other collectors were the only ones who got to see this impressive collection, which fills his home's entire third and fourth floors. Then a few years ago, Mr. DeSantis decided to open it up to the public as part of the Old Allegheny Victorian Christmas House Tour, which this year runs Dec. 12 and 13.

Some tour-goers might balk at paying an additional $10 to see the collection but, considering it's one of the world's largest private toy train museums, it's worth it. Train lovers also get to see the elegantly appointed second-floor bedrooms that are off-limits to other tour-goers.

"The Great Wall" of Ives trains is but one of many highlights for visitors. Another is a single room on the third floor that houses more than 100 one-of-a-kind pieces, including the prototype for the first Lionel locomotive in 1906. Also displayed is a sloped-back Lionel "tender" prototype from 1915 that never made it to market.

A room across the hall boasts a rare Blue Comet set made in 1929 by Boucher, considered the Tiffany of toy train manufacturers in the 1920s. Fewer than 12 of these sets, which measure more than 9 feet, are in existence, according to Mr. DeSantis.

Standard gauge trains, whose rails are 21\u20444 inches wide, fell out of favor in the late 1930s because many houses simply didn't have enough room for the big metal cars, which were about 20 inches long. Mr. DeSantis also has many fine examples of smaller O gauge trains (11\u20444 inches between the rails). One of the more unusual pieces is a Dorfan train on a circular track that originally was used as a dealer's display.

Rather see them in action? The fourth floor holds more than a quarter-mile of track with working layouts organized by manufacturer (American Flyer in the middle, Ives on the right, Lionel on the left and Dorfan in the rear). Grandsons Shane, 5, and 3-year-old Riley play with the trains every day after school. Just as fascinating are the train accessories. Everyone expects houses, trees and people next to the rails. But how about a silk-skinned Zeppelin made in 1929 by Erector or a cast-aluminum monorail system by Aero that dates to the 1930s? And don't miss the replica of the Duquesne Incline built from two Lionel observation cars or the hand crafted scale model of Conneaut Lake Park's Blue Streak roller coaster that Mr. DeSantis found, curiously enough, in Atlanta.

His collection also includes dozens of pieces related to "real" railroading in Pittsburgh, including the neon sign from the roof of the B&O Station, which graced Grant Street before being demolished in the '50s to make way for the Parkway East. Then there's a metal ticket window from the Fort Wayne railroad station on the site of 4 Gateway Center. He also has a "Track 1" sign from a train shed that stood at the end of the Wabash Bridge on the Monongahela River.

Collecting toy trains, says Mr. DeSantis, is a wonderful hobby, in large part because you're playing with toys.

"You'd be surprised how many people had them as kids," he says. "It's like playing with dolls in a doll house, except for boys."

The guided Old Allegheny Victorian Christmas House Tour runs every 12 minutes from 5 to 8 p.m. Dec. 12 and 10 a.m. to 8 p.m. Dec. 13. Tour tickets cost $20 and include several restored Victorian homes, plus Calvary United Methodist Church. There also are brunch and lunch tours ($40) and a five-course dinner and tour ($100) on Dec. 13. Tickets are limited and can be purchased online at www.alleghenywest.info or by calling 412-323-8884.