Trains come north and trains come south. they stop here -- 4,300 freight cars a day from six different railroads -- somewhere along these 93 tracks, 110 miles of rail known, as the Potomac Yard, to be broken up into new trains heading in other directions. This is the largest freight "classification" yard between Philadelphia and South Carolina, a 6 1/2-mile stretch from Crystal City to Alexandria owned by the Richmond, Fredericksburg & Potomac Railroad.

To classify a train is to "hump" it. At the center of the yard stand two rises -- humps of ground. Inbound trains are fed over them, and as each car crests the hill it is cut free. Gravity does the rest. The loose cars sail down off the humps and out into the classification yards where they are switched onto various tracks. A different track for a different destination.

Next to the southbound hump a sign reads: Safety FIRST, and now, with night coming on, those words glow orange. "This can be a damn dangerous place," says one man here. "It used to be that a man rode each car off the hump. In the '40s there was no light down there; you'd have to look for the shiny part of a wheel or something. Helluva lot of people killed out there."

Machinery has removed some of the danger. The cut cars travel through a section of track that automatically brakes them, lets them ease on down into the yard. There is no longer need for a man to scramble onto every car and crank away at the handbrake, to squint in the blackness for the car ahead before the one he is on rams into it.

Look through the night for a faint light, for lamp swinging from the hand of a man stepping over track. This man's names is P. D. Halsey, and he is a conductor. His job is to take all those free and loose cars in the classification yard and, working with his engineer and brakeman, "make them up" into new trains. They may have to work as many as six different tracks to make a new train, backing and filling the engine onto each, coupling 15 cars here, 20 cars there.

There is an art to this. The idea is to figure sequences, "line your switches up." Switches occur in a staggered pattern across the yard. Glowing red and green, they are called "targets." Halsey has to figure what order he wants to throw them to open the track to the engine, so he can cut down on wasted time, motion and distance. He has to remember the relationship of his switches, the growing size of the train, and that buried deep there may be a car that has to be cut from the herd and sent over to the shop.

Halsey, a conductor for 19 years, stands at a closing gap of cars an speaks to his engineer on his radio. "Half a car-length now, Scotty." The cars groan, the ties creak. A boxcar these days weighs 40 tons and can carry twice that. A train might consist of 125 cars. The cars now couple with a lurch and a boom with travels on back through the train like a seismic wave. Halsey steps in, kicks a "skate" out which has been blocking the wheel of the lead car to keep it from moving, and clanks around with the cut lever to asure the couple. "Go north, Scotty," he says, and the larger train rumbles ahead.

The joined freight cars have closed off the space of night and left a narrow cavern between a pair of loaded tracks. Halsey walks south, heading to skirt the train an cross over to another track to throw another switch. He walks three or four miles a shift.

He says back over his shoulder: "You learn quick to keep your eyes open. You watch out for the unexpected. If not, you can get yourself cut up fast down here." As if to emphasize what he says, the cars on the adjacent track suddenly move, bumped unexpectedly by a car or two that has been sent down off the hump. Now with both tracks in simultaneous motion, the effect is dizzying.

Last winter a man down in the Southern Railroad yard just south of here stepped between two cars to couple their air brake hoses. He had 35 years' experience and had made that move thousands of times. The track was icy. He happened to slip, the cars happened to move. He as sawn in two lengthwise.

Halsey comes across a mound of coal spilled from a car. "You get stuff like this lying around." A little further on he walks over some scrap wire. He kicks at it, and for a moment it gets tangled in his feet. "It doesn't take much to trip and . . ." He doesn't finish the sentence.

The reality of one man meeting the danger posed by one machine has a dated feel to it, for America has chosen to live with risk that comes in more random, more volatile doses. About 40 chemical cargo cars spice the daily traffic here. One night awhile back a boxcar full of toilet paper caught fire, and when the fire company came to douse the flames they found the water pressure a comparative dribble. Luckily, the car wasn't a tanker and the fire wasn't to searingly chemical that they couldn't nudge it closer to the water source.

John McGinley, the yard superintendent, says the yard gets its water from the city of Alexandria and that "the city couldn't provide us with enough water to satisfy the current [fire] standard."

Alexandria Fire Chief Charles Rule says, "I have to agree. The railroad has shared an unfair burden of the blame." He points out that Alexandria does not own its own water system but contracts with a private company for service, a company that oversees a system built a century ago and has made few capital improvements. Meanwhile, America has been producing 50 million metric tons of hazardous waste a year and shipping a lot of it through urban centers. Rule says the railroad is now building pumping facilities to help beef up the water supply "The water company," he continues," is moving to update an outdated system." The immediate responsibility is now, somewhat belatedly, being met by the fire and water companies, by the railroad. He feels, though, that it should not stop there. "Our society seems to need this stuff to survive." If that is the case, he says, then society should understand the consequences.

John McGinley says: "There used to be farmland coming right up to the edge of the yard. Back then we drilled our own wells and we could meet the fire standard." Now, a quick glance around will tell you that the setting is anything but pastoral. Above ground the 20th century and its technology boom along. Below ground, silent, rusty and out of sight lies the 19th.

There is something about a freight train that stirs the blood. Racketing past, it is a linear mosaic of colors, names and slogans. The railroads share their cars, so the diesel of one line might pull the cars of 50 others. It pulls cars that have climbed western mountains, cars that have crossed the sandy coastal plain, cars that have ridden the prairie with the wind.

In this yard there is that polyglot feel, that feel of wideopen, bountiful country. There are big names you have doubtless heard of, and there are names you haven't: The greenville and Northern, The Meridian & Bigbee, The New Hope & Ivy Land, The McCloud River -- lines that travel into the far reaches of the continent, in this land of wanderlust, into the far reaches of the imagination.

That is the romance; the reality is something else. Wallace Piety sees that because he is a policeman here; he walks trains when they come in, looking for open doors. Sometimes Piety finds people. There are, he says, the older lifelong hoboes who lives, in a sense, purposeful lives. The younger men, though, seem a different breed, "coming from no place and heading nowhere." The youngest he ever encountered was 14, a runaway from Florida.

The older men Piety sort of knows. "You see the same faces every four to six months." Like the birds, they migrate north and south with the seasons. There is a man who was a certified public accountant in New York, and one day packed it all in. Another from South Carolina felt too much grief at the death of his wife. A handyman who commutes to Baltimore on weekends to visit his brother.

Piety grew up in Florida. He recalls standing on a hill near his grandmother's house with his boyhood friends. As the engine labored up the hill, steam pouring from its wheels, smoke from its stack, they would cheer it on. He remembers being awed.

When he goes into the yard, Piety says a "sixth sense" guides him, a sense whose bedrock is the five others. He feels that the night has sharpened his vision, and that his nose can pick up such signal odors as an unwashed body or an open jug of cheap wine. Sometimes he walks on the ties, opening up the air for his ears to pick incongruous sound in the hollow of a boxcar.

But when Piety goes into the yard his senses are tuned as much to property as they are to people. One of his jobs is to replace seals on door latches that have been misapplied by the shipper, thus leading to the possible damage or loss of cargo. He sees such carelessness as symptomatic of our no-fault society. "I think this is a big ripoff. The companies always know they can file a [insurance] claim against the railroad." He sees more coal going through, but he sees it going both north and south. If supply and demand exist in the same places why, he wonders, is there need to ship between the two? He senses the slowing economy -- fewer automobiles being shipped, especially Cadillacs. Its fancy radio was a target for thieves and Cadillacs showed up with slashed seats, broken windshields, stripped to their wheels. The factory complained. The railroads said they could only offer so much protection. Nowadays Cadillacs ride by truck.

There is a single, dominant sound in the air here. It is shrill and piercing, almost a shriek. It is the sound of wheels braking on rail, of steel against steel. In an age of plastic that sound is anachronism.

It is like the roundhouse here, all but a relic from the age of steam. When diesels came in the late '40s it meant more power, less maintenance. Crafts built around the coal fire -- boilermaker, fireman, blacksmith -- began to die.

E. G. Rogers is one of two blacksmiths left at Potomac Yard. Before him lies an array of his labor: cut levers reworked, center pins straightened, car ladders resoldered -- made whole again. If a car ladder gets broken or bent the implications could be fatal. Somewhere down the line a man reaches blindly for the ladder on the side of the moving car that he knows is always there and grabs only air.

On the walls hang Rogers' tools -- black medieval-looking implements. A tongue of flame, fed by oil, roars at the forge, and next to that sits a rusty bath of water into which the blacksmith plunges overheated metal.

The light is grainy; the iron-dark shapes of the shop contrast with the light of a balmy March afternoon flowing through wide-open doors, where Rogers goes to get away from the ferocious roar of the fire. He recalls how once the fuel was coke and how the men built their own fires out in the yard and hauled their equipment from job to job. "It was all muscle and blood then."

The work was harder, the pay less. But then, too, there was more need for that work. "The blacksmith's about gone. They buy a lot of these pieces new from the factory now instead of fixing them." Rogers' biggest gripe is that the company doesn't buy him enough tools. "They don't want to give you what they've [already] given you. I like to do things real neat, and you need good tools to do that. And they always want the cars [fixed] right now. There's never enough time."

Rogers has heart trouble and soon he will retire. When he does he wonders if, in his case, "you take your job with you" -- if the company simply will not refill it. He turns back from the light and the spring air and that shrill, everpresent sound of the rails singing in the yard. He confronts the ladder he is working on, laid out before him like pieces to a puzzle. Every man needs as ladder he can reach for somewhere on down the road.