The day train south from Cuzco, Peru, carried us down toward the place where the continent now cleaves politically. It was a slow train, crowded and warm. It began in valleys cut like moist spice cake, with oddly shaped patches of new corn and potatoes. From time to time, it bumped to a stop at a clay-colored village where the roofs were mostly thatched.

At the stops, wide-skirted women moved down the aisles selling bread, or hats of thick white fur, or boxes of brass bells that would ring out suddenly over the stream-hissing clanking of the railroad cars.

The air turned chill, and as the waiter sat down plastic bowls of vegetable soup, a light rain began streaking the train windows. The pregnant woman from Lima shifted in her seat and looked up when a raindrop hit her face.

The colors changed. The ground was the flat gray-gold of the altiplano, the high plain, with low mountains sharp in the distance and brown vicunas cantering alongside. The towns hugged the earth now from inside their long walls, and outside it was spare, spindly, sad, and beautiful. A lone woman walked, lifting her legs high. Her orange skirt was the only touch of color on the plain.

I HAD EARLIER flown out of Bolivia to get rid of certain notes and documents, after the eighth person told me our hotel room might be searched. The material was in a suitcase in Lima, checked into a hotel luggage room, and now the train rocked back down toward the border and the great lake, Titicaca, that straddles Bolivia and Peru.

The lake is said to be the birthplace of Inca culture. There is an island in the midst of its deep blue waters where tourists descend from hydrofoil boats to look at the solid stone steps of a planned agricultural society more elaborate than any other in the ancient world. Even before the boat touches dock, island people are trotting down the steps with small reed boats they will trust at the tourists, whimpering sofely in Spanish, "Buy from me, please?"

There is also a church on the Bolivian side where statues of the 12 Apostles once adorned an outdoor arch. Nine of the Apostles have fallen. When the last three fall, the world will end.

It is the moment of utter astonishment that will stay with us now, long after the rest of that train ride has gone, the realization that two Peruvian leftists were about to begin telling North American strangers what they thought of things. They were smiling. They were articulate. And they were speaking without fear.

THERE IS A BARRIER now in South America for foreigners steeped in a particular tradition of free speech and ideological pluralism. It splits the south from the north, and we were pointed straight at it in the evening on the Cuzco train. In Argentina, one does not discuss certian topics on the telephone, or without a trustworthy contact -- Marxism, for example, or the labor movement, or the people who were taken away in the night.

In Paraguay, a taxi driver had explained with some pride that every Paraguayan was a policeman for the president. In Bolivia, I held a low-voiced conversation with a professional couple who kept their eyes on the door to the office where we met.

The couple had described what they believed were paramilitary assaults on private homes, and when the office door opened suddenly, the wife had smiled brightly at me and raised her voice. "Have you been to the Indian market?" she had asked. "The medicinal herbs they sell are quite interesting."

Campesinos (peasants) had met me in a faded La Paz hotel lounge and spoken of military encampments in their towns. The instant a stranger passed by, they began to talk about sugar production. Their voices never changed.

Once a campesino had talked about coffee for a full 10 minutes while I sat, glazed and smiling, waiting for the stranger to leave the room.

"Gorillas," said one of the Peruvians. He was talking about the Bolivian government. His face was growing blurry in the dim night light of the moving train. It was an Andean face, dark, Spanish mixed with Inca, the color of wet Adobe. Gen. Luis Garcia Meza, the new president of Bolivia, has a similar face.

IF YOU WERE dividing South America by logic instead of national boundaries -- the way Simon Bolivar tried to do it during his ultimately fruitless search for unity among the Spanish colonies -- you would put one of your lines around both Bolivia and Peru, around the Andean nations, around the mountains and the vast populations of conquered Indian people who still speak Quechua and Aymara, languages the Spanish never understood.

Argentina, with its Italian immigrants and its obsession with European culture, might as well be a continent away. "Really, we are not like the rest of Latin America," the Argentines will say.

Professionals say it with a faint touch of embarrassment, as though wincing at the idea that the pale evening commuters on the Buenos Aires subway should have anything to do with a Bolivian campesino's struggling patch of corn. And yet Argentina has wrapped Bolivia into its cool official embrace so that they both now use the ideological battle language of the southern cone.

"We will combat subversion in all its forms," goes the common battle cry. Both nations publicly deny extensive reports that Argentine advisers helped direct the Bolivian coup. By the time you have listened to a Bolivian official describe the Process of National Reconstruction, in careful language almost identical to the Argentines' both assertions and denials seem almost irrelevant.

The ideological border of the southern cone has moved north to include Bolivia. It lies now between two Andean nations that gaze from opposite directions at the same high plain, and only on one side of it -- the side that has not defined subversion to include the uninhibited exchange of ideas -- has a stranger spoken openly to us about his own dissenting notion of how the nation ought to be run.

I CROSSED INTO Bolivia again and took photographs in La Paz of the closed university, the open market on steep crowded cobblestone, the half-destroyed workers' headquarters and the troop carriers rumbling down gray brick streets.When I opened the camera on the last morning, the film was gone.