YOU'RE DOING what?

Everybody -- Mexican or American, tourist or resident -- everybody said that when we told them we were taking the train from Mexico City to Oaxaca. After all, it takes 45 minutes to fly. And 15 hours on the train, winding through the mountains almost all the way. You must, they all clearly thought (and a few actually said so), be nuts. They were wrong.

The trip was not luxurious, mind you, nor even terribly comfortable, even though our reservations had been made weeks before and we were in one of only two first-class sleeper cars in a half-mile-long segmented worm, squirming around and around the bases of the desert mountains that scar the midsection of the Mexican interior.

But that was almost irrelevant. Because the train takes you through the worst of Mexico and the best of Mexico. You are a part of it, yet you are apart from it. It is a 3-D ever-changing, live tableau.

No bus, no plane, no automobile highway takes you through the parts of the country penetrated by the train. Perhaps half of the trip is spent asleep, and for the rest, one is glued to the windows, alternately appalled and transfixed.

The sleeper cars are old U.S. compartment cars replaced long since in this country by newer models. They are all equipped with buttons for air conditioning and temperature control, but of course in Mexico, none of these buttons is hooked up to any power source. You're lucky to have a dim lightbulb and a noisy, rather ineffectual fan. The windows don't open, and what air you get comes in through the openings at either end of the car. There is running water, but one does not dare to drink it, any more than one drinks running water anywhere in Mexico.

The train lurches out of Mexico City about 5 in the evening, and immediately you are stricken by what is to be the most grueling -- and powerful -- experience of the train ride.

The train makes its way slowly to the edge of town, through some of the most pitiful, sordid, (and seemingly endless) slums one can imagine.

Shanties are built on rooftops, and on the roofs of the shanties are built even more shanties. If a train car is left standing on a siding, a family or several families move in and set up housekeeping.

Barefoot babies play apathetically in the dirt. Dogs, as scrawny as the children, root in endless mounds of garbage. The Mexicans have an expression, "Cada hijo trae su pan abajo su brazo (every new baby carries its bread, its sustenance, under its arm into the world)."

It is a sad and bitter untruth. And when there are too many mouths to feed in the countryside, the family moves toward the city, where whole families sit begging or peddling cheap made-in-Hong-Kong gewgaws on the streets and in the parks of downtown Mexico City. Groups of children, working for an adult or an older child, accost tourists with wide eyes and handfuls of Chiclets. They keep virtually nothing of what they earn.

Time and Mexico's rocky economic status have changed this situation only for the worse -- if that were possible. The image of the drawn, dirty face of a solemn 7- or 8-year-old, bending under the slight weight of an infant carried in her voluminous, ragtag rebozo, watching wide-eyed as the train passes remains to haunt.

The slum is not new -- only newly discovered by those who would highlight the current state of Mexico's chaotic economics.

It is only with the coming of night that the train is buffered from a misery one is helpless to alleviate in even the smallest degree.

But when the darkness lifts it is in a kind of horticultural paradise, and although the sights of the night before will remain indelible, they are briefly cleansed in the purity of desert spring.

Every weed, every cactus, every sort of spined, thorned, prickled growth is taking advantage of the briefly benevolent clime and presenting its flowered face -- from tiny white stars to the Flamboyant's aptly known cerise. The most precariously rooted cacti in the most arid-looking outcroppings are blooming with wild abandon.

As the train continues to thread its way through these long-dormant volcanoes, it crosses a wide riverbed where prudent farmers have planted fields of corn or of vanilla beans or cacao plants (or -- but out of sight -- cannabis plants) alongside the trickle that will, later on, become once again a roaring torrent, burying the crop land. But for now it is a fertile field, and for this season it too blooms.

It is barely dawn, but these tiny fields are already being tilled by the families who have not yet been driven to the city in search of sustenance. Their huts are perched, often, as precariously as the ubiquitous yucca, defying the rocks, gravity, earth tremors, clinging with blind tenacity to their piece of the planet.

The train stops periodically, now, and one or two people get off at each stop. It is an event, apparently, because entire villages seem to have gathered to noisily and lustily welcome the travelers home. It is barely 6 a.m. But with all the stopping and starting, the train manages to adhere, more or less, to its schedule.

It is the holy Easter weekend and the second-class cars are jammed almost beyond capacity. A sombrero-ed farmer boards at one of the small mountain village stations and swings his sack off his shoulders to a pile of suitcases and packages. The sack immediately takes on a life of its own, bulging this way and that in a dozen places. It is filled with baby chicks for sale at the Oaxaca marketplace.

Two American women, unable or unwilling to pay the obvious bribes seemingly required in Mexico City to obtain first-class tickets -- or second-class seats -- have been standing most of the night. On their own, without benefit of travel agent or tour guide, with little grasp of the language and no concept of the ubiquitous corruption that is a special burden of the Mexican poor and that accounts for at least a part of today's economic chaos, these women were alternately enraged, appalled, frustrated, resigned and ultimately amused. Despite their 15 hours of more or less acute discomfort, they are, at the last, also transfixed by the scene.

There is no water and no sanitary facilities in the second-class car--they were locked by the conductors. During the night a group of Indian women passengers, all small, all loaded with assorted straw (and plastic) receptacles, have crept under the filthy train seats to sleep on the car's even filthier floor. At each of the village stops people selling tacos, hot, redolent coffee and cold bottled Cokes will get on, peddle their wares and jump off the next time the train slows, sometimes apparently in the midst of nothing.

There is no dining car on the train. We found a supermarket about a block from the station in Mexico City and stocked up on fruit juices and crackers and cheese. But it is not a trip that whets the appetite.

The train bed is less than smooth -- each lurching stop and start brings unbidden thoughts of earthquakes, volcanic eruptions or worse -- and it is clear that second class is not to be considered a viable alternative to anything.

Nor is even the first-class trip something to be repeated at short intervals.

But it brings insights of its own. Unique and unforgettable.

As one Mexican put it, "That 15-hour train trip puts you closer to the real Mexico than anything else in the country."

He wasn't recommending it, however.

Freelance writer Lisa Northcutt, who traveled second class in the front car of the Mexico City-Oaxaca train -- but who probably won't ever do it again -- also contributed to this article.