"Look at that son of a gun now. This is bad," said electrician Earl Shaffer as the freight train rumbled past. "See? He won't stop there."

Shaffer was perplexed: The train should have stopped at a signal light. "That way people would know the signals were working," Shaffer explained. "Otherwise they would say they are just lights." And his main job at the B&O Railroad Museum is seeing that every little thing in the H-O layout works just right.

Model-train buffs who visit Baltimore's big rail museum head for the H-O layout, but there's much more to see at the train station at the corner of East Paca and Poppleton Streets. Another layout, a circus, is very detailed, with wheels that turn and cages that open, but the circus trains are stationary. On loan to the museum through January (and possibly longer) from its builder, a West Virginia man, it draws the circus modelers. And then there are memorabilia from railroading's earliest days to the present, and, of course, some big trains.

Part of the museum, the Mount Clare station, was the nation's first passenger depot when it was built in 1830; another, the roundhouse, was completed in 1884 and restored in the early 1970s. It houses many of the museum's 53 engines. Locomotives from recent years overflow into the front and back yards. Alongside some of the trains are platforms you can climb on to look inside the old Pullmans and dining cars. There are trains from the Civil War and a "40 and 8," a French boxcar decorated with provincial coats-of-arms. Used in World Wars I and II to transport men or horses, its capacity is listed over the doors: hommes 40, chevaux (en long) 8.

You can walk through a postal train or a caboose, or stand in the cab of yet another train. Young children tend to favor these, but so do the basic rail buffs who come to take pictures. There's also a replica of the first steam locomotive built in America, the Tom Thumb, which has given rides to visitors on summer days. It may run again if volunteers find they're unable to refurbish a new acquisition, an engine that formerly hauled coal to St. Elizabeth's Hospital. A smaller Tom Thumb engine encased in clear plastic can be operated by pushing a button. "That's why they call it Tom Thumb," one 12-year-old girl observed, "because you press it with your thumb."

It's hard to explain the esoterics of model trains to someone who isn't a train buff, though Shaffer tries. He's more comfortable talking to the hobbyists who gather around the H-O layout. "Mostly what I use here are GP40s, because they give me such doggone mileage," he said to Jim McGowan. A 37- year-old management analyst for the Social Security Administration, McGowan has taken up modeling just in the past few months. "A lot of 35-year-old kids come here and look at the stuff," he said. His father worked for the B&O railroad, so he's been interested in trains since he was a boy, but never had anything on what he calls "a realistic scale" before.

As the recalcitrant train edged ever closer to the caboose of the lead train, Shaffer made up his mind. "I'm going to have to put more cars on," he said decisively.

But later, when the freight train was pausing at its mark, the nuances were lost on some of the children. "This one stopped again, James," said a little girl named Laura to her older brother. "Yeah," he said knowingly. "It always gets stuck there."

Out in the front trainyard, where things are life-size, Harrison Sayre was taking a long look at the lean Capital Limited, Art Deco-style in metallic blue-gray. A sign designated it as the first electric diesel. "I remember riding on that," he said, in the early '50s, when he worked for the Navy Department. "The Royal Blue, the Capital Limited, the Shenandoah. The Ambassador used to go out to Detroit. . . " This day he'd driven over from Annapolis to see the model circus show in the museum's roundhouse, because he builds model circuses -- for him a kind of spinoff from model railroading.

His love for trains all started when, as a boy, he lived in Schenectady. "Our house was a couple of miles from American Locomotive Company, where they built the locomotives at that time. I remember going by there and seeing them out and going through the plant."

"These rail fans," said Marian Smith, a person who sees a lot of them because she manages the museum. "Maybe when they were children and saw trains going by their house -- maybe if you're small and see this huge train going by, look at the power, and the smoke -- it just becomes a thing that stays with them all through childhood and into their adulthood."

A trio of rail fans sat by the turntable of the immense roundhouse, near the model circus and close by the William Mason, an original built in 1856. Milton and Betty Woodlen of Phoenix, New York, had come with their friend William Landrum from Westchester, Pennsylvania. Milton Woodlen models circuses and had just put some of his collection on display in the public library back home.

About this model circus he said: "The workmanship is very good and the scaling is excellent. What happens with so many is the builder will get pressed and then blow it. Usually they will make the tent poles too thick. . . But this one is in perfect scale as far as the eye can tell."

There was not just a Big Top, but a sideshow tent, a menagerie tent for the animals and a dining tent where circus workers were gathered. There's tiny silverware, but builder George Neal is still cooking up the food for each tiny plate. Neal, who works for the West Virginia Employment Service, has been modeling circuses for more than 50 years, and this particular one since 1969.

The Woodlens' friend Landrum, a retired English professor, expresses his love of trains differently: His hobby is riding the rails. "What else is there to do?" he asked. He's been on the Trans-Siberia train, eight days from Moscow to Vladivostok, which he recommends along with the Blue Train from Capetown to Pretoria, or the Indio-Pacific, which reaches from ocean to ocean, or the Indian Express from Perth to Sydney. "You get into a compartment and you actually see the country before you," he said. There has always been someone who speaks English -- even in the mountains of Siberia, he said.

But he doesn't plan to write a book on his experiences, as others have. "If you did it for some other purpose than just the joy of it," he said, "you wouldn't get as much pleasure out of it."

THE B&O RAILROAD MUSEUM -- At 901 West Pratt Street in Baltimore. Open Wednesday through Sunday from 10 to 4; closed on Mondays, Tuesdays and major holidays. Admission: $2 for adults, $1 for children six to 12. Group rates available. Call 301/237-2387.