Carrying babies in backpacks and all-day supplies of food in picnic hampers and coolers, they converge on Baltimore's Amtrak Station several hundred strong. They wear Oshkosh B'Gosh bib overalls and denim jackets adorned with badges from every passenger line that ever went out of business: the New York Central, the Erie, Jersey Central . . .

Some sport bright yellow hats that tell you where they're going - "Chessie Steam Special" - a series of rail excursions pulled by an old-fashioned steam locomotive.

On the train, an assemblage of 21 reconditioned railway cars of varying ages, it's hard to tell the train buffs (called railfans) from the Chessie personnel. Finally you figure out the most of the Chessie people are dressed in business suits, while the railfans are the ones wearing conductors' hats and brass trainman badges.

In each car is a host, a member of one of the local railfan clubs, who's in charge of safety and spinning railroad yarns. "We're asking people not to sit here yet," says the host in the open-sided sightseeing coach, a former New York Central sleeper rebuilt and fitted with the kind of hard wooden seats they have in third-class trains in South Korea. "We're still in the missile range," meaning that Baltimore kids have been known to throw rocks at trains. A few miles out of the missile range the train stops and the diesel locomotive that pulled us out of Baltimore is disconnected. In its place goes the raison d'etre for the trip: Reading No. 2101, a steam locomotive that pulled freight trains from 1945 until 1955. Saved from the scrapper's torch in 1975 by New York businessman and railfan Ross E. Rowland Jr., it pulled the American Freedom Train during the Bicentennial year.

Rowland is aboard, but not in his customary spot. Having separated his shoulder the night before, he has his arm in a sling and can't drive the train. Instead, he sits in the observation lounge, which he also owns.

"My father was vice president of the Jersey Central Railroad," says Rowland, "and he used to take me out on the road with him in his private car, No. 97. I've always loved trains."

When he was six, Rowland was hanging around the railroad yards in Cranford, N.J.; by the time he was 10, the crews were letting him drive locomotives around the yard. School held little attraction for him and after 10th grade he left and became a runner on Wall Street.

"Then I got lucky," says Rowland, now 37 and a member of the commodities exchange. "We hope excursions like these would cover the costs, but they never did, so I subsidize them. My gratification is driving that locomotive. Do I mind the soot and grime? I love it. That's the closest thing to haven there is," Rowland says, cursing his shoulder injury.

The locomotive whistles and puffs and we chug through Maryland farm country toward Gettysburg. At Emory Grove, we enter the "Dutch Line," or Hanover subdivision of the Western Maryland Railway, which is still heavily used for freight traffic. The countryside has rarely seen anything like this. People with cameras, children and picnic lunches line the banks. Cars follow the train, slowing down to keep pace. A herd of cows, scared perhaps by the cloud of black smoke and the moaning whistle, moves hurriedly away from the tracks.

Up front, in the recording car, railfans tape every moan of the whistle, every turn of the wheels, every puff and chug. A former mail car with big window openings, the recording car is equipped with outlets for tape recorders.

"I'll condense it down to what I want," says a railfan with a tape recorder. "Then I may trade it to someone who has a tape of a trip I didn't go on."

"Is it saying 'I think I can, I think I can'?" asks a little girl who has read the book about The Little Engine That Could. The railfan hasn't read the book and looks at her blankly.

Just behind, in the open-window coaches, other railfans are swapping yarns and wearing goggles, which aren't an affectation. A boy sticks his head out the window. "I got a face full of cinders," he says with great glee.

At Gettysburg, near the spot where Lincoln once got off a train to make a speech, the tenders, which carry the water that makes the steam, must be refilled. Together, the two tenders hold 35,000 gallons of water, which the Gettysburg Fire Department draws out of a hydrant.

"Since there are no more steam trains, the railroads don't have water towers anymore," explains a railfan as a Chessie official tries to get passengers who are continuing on to Hagerstown back on the train. There are tour buses for people who want to get off in Gettysburg and catch the train on the way back, but the true railfans ride on.

A few miles past Gettysburg there's a photo run-by. Everyone gets off the train, which backs up and steams past the picture-takers. The first run-by is slow but the second is a fast 35 mph. The American flags flap in the breeze. The engineer waves. "Now we know what we look like!" says a passenger.

A little farther on the diesel engine that pulled us out of Baltimore, which has been running ahead of us all day in case of emergency, is put back on. "We're double-heading," says a railfan, meaning that both the diesel and the little engine that thought it could are pulling us over Jack's Mountain, a steep grade. Once over the mountain, the diesel is detached and we steam into Hagers-town for an hour layover.

"This is a luxury, wandering around a yard like this," says an ecstatic railfan. "They usually don't let you on the property because the liability insurance is so high."

Railfans crowd around No. 2101, watching the crew pump pin grease into the side rods of the 414-ton locomotive. A crane is loading 26 tons of coal into the tender while sand is being drawn out of a tower into a spout on the engine. This is called sanding, explains a railfan. The sand goes onto the track through tubes in front of the wheels for traction. Meanwhile, up in the locomotive itself, Dave Rider, the fireman, has already started the fire that will make the steam that will carry us back to Baltimore. He opens the doors and shows a red-hot parochial-school vision of hell. "We burned almost all our coal on the way up, and we'll probably burn another 26 tons on the way back," says Rider.

Steve Ventulis, the engineer, his face streaked with grime and his classic engineer ha and kerchief heavy with soot, is checking all the gauges. "The speed limit on this line is 50 mph, but on this run we never did over 40," he says.

The passengers get back on the train, which still must be turned around the watered. The latter seems to take an inordinately long time, probably because they are using what one railfan refers to as "a garden hose." "I remember when the New York Central would stop at Harmon," he sighs. "They could fill a large tender in 10 minutes."

On the open-air sightseeing car, railfans with soot-streaked, tired but happy faces look out over the Hagerstown yard. On the track next to us is a cylindrical glass-lined tanker used by Gallo to ship wine. "I wonder if it's full," muses a railfan. "We could just unscrew the cork at the bottom. Then, I wouldn't care if we had to stay here all night."