ON BOARD THE TANZANIA-ZAMBIA RAILROAD -- There are Mao-suited Chinese men out on the loading platform, grinning and shaking hands. Next to them, in a squall of chirping and yellow fuzz, Zambian farmers load day-old chicks bound for Tanzania. Third-class compartments are packed jowl-to-elbow with Africans -- nursing mothers, sleepy old men, big-eyed little boys. In first class, a Zambian businessman with diamond cuff links and a cassette player has cranked up a Beatles song, "Love Me Do."

Prowling around the platform, amid the Chinese and the chickens and the teen-age girls hawking green oranges, are plainclothesmen. No photographs allowed. They are sniffing after South African saboteurs.

At 11:15 on a Friday morning, right on time, horns honk, the Chinese wave goodbye and the train pulls out for its 1,161-mile run from the central Zambian town of Kapiri Mposhi, about 100 miles north of Lusaka, the capital, to the Indian Ocean.

If South Africa were to impose retaliatory economic sanctions on the black-ruled states of southern Africa, the Tanzania-Zambia Railroad, or Tazara, is positioned to become the sole lifeline for Zambia, Zimbabwe and Malawi.

Those front-line states now use transport routes through South Africa and rail links to ports in Mozambique. Pretoria can close the former whenever it wishes, and the latter are vulnerable to sabotage from South African-sponsored Renamo rebels. The one remaining rail route to the sea, Angola's Benguela Railroad, is hostage to UNITA rebels, whose principal patron is also Pretoria (a secondary patron is the United States).

The Tazara is more than 1,600 miles from Pretoria, the South African capital. That distance has made this railroad a strategic component of European and, most recently, American plans to ensure that South Africa cannot strangle the front-line states.

This 11-year-old railroad once was labeled a $500 million mistake. The World Bank, the United States and a gaggle of European experts said it would be a waste of money, that it would not make a profit and that, in any case, it was not needed.

Three leaders, two African presidents and one Chinese Communist Party chairman, disagreed. They wanted a "freedom" railroad that would end Zambia's dependence on white regimes to the south.

In the late 1960s, when Tanzania's Julius Nyerere and Zambia's Kenneth Kaunda could not coax any money out of the West, they turned to Mao Tse-tung. None of the three had ever had much luck running their respective countries' economies, but as it turned out they knew how to build a railroad.

The Tazara now carries nearly all of Zambia's copper exports, at about half the cost of the next cheapest rail outlet. It attracts far more passengers than it can carry. It has opened up the rich but inaccessible highlands of western Tanzania to a flurry of development. The train is slow and prone to accidents, and it mashes an occasional giraffe. But it usually makes money.

European countries have committed $45 million to a 10-year project to rehabilitate the Tazara and increase its freight capacity. An additional $68 million is under negotiation. The U.S. government, according to Tazara officials, is in the final stages of approving about $36 million to supply 14 American locomotives and maintenance equipment. U.S. officials confirm the plan, but refuse comment on the amount of money involved.

Human cargo has never been and will never be the priority of the railroad men who run the Tazara or the antiapartheid donors who give it money. There are five freight trains a day, but only six passenger trains a week. Donors are spending millions to help the front-line states move copper, fertilizer, oil, grain and spare parts -- not people. Tazara officials have asked donors for 1,050 new freight cars; they don't want any more passenger cars. The movement of African freight subsidizes the movement of Africans.

"Passenger traffic is only a social service," says Standwell Mapara, general manager of the railroad.

For Africans, however, this social service, this afterthought to the delivery of fertilizer, has transformed the Tazara into a kind of magic carpet for the common man. In much of Zambia and almost all of Tanzania, reliable, affordable and relatively speedy long-haul public transportation is otherwise nonexistent. This Chinese train is all there is.

More than 1.3 million people rode the Tazara last year. Perhaps twice that number would have ridden had there been room. While western powers want the railroad to carry more freight, Africans demand more seats.

The romantic allure of train travel, an anachronism in the United States where it has been mummified in ballads of lonesome whistles and faithless love, is alive and well on the Tazara.

This train is a mesmerizing curiosity. Across Zambia and Tanzania, people stop and stare at it with wide eyes and open mouths. Unlike anything this part of Africa has ever known, the Tazara is a regularly scheduled connection to a world beyond thatch roofs, dirt floors and long nights with no electricity.

The journey from central Zambia to Dar es Salaam, the Tanzanian capital, is farther than from New York to Kansas City or London to Moscow. It brings together Africans and a few foreigners who normally have nothing to do with each other: monied businessmen and subsistence farmers, assorted smugglers and soldiers on leave, aid workers and Scandinavian rucksackers and Africans on the lam.

Up to 2,000 people wedge themselves into 12 passenger cars for the two-day ride. There isn't much to do but hold your seat, look out the greasy windows and listen to the voices of Africa.

It is cheap to ride the train. A one-way fare is around $10, third class. But it is not pleasant, particularly in third class, where 88 percent of the passengers ride.

There are 93 station stops. Many third-class passengers stand up all night or sit on each other. Compartments designed to carry 96 people usually carry about 300. There is neither water nor food service in third class. The toilets are unspeakable.

Being rich for a Zambian, Pius Kakumgu always rides first class. Since most Zambians and Tanzanians are not rich (annual per capita income, respectively, $390 and $290), Kakumgu has no trouble booking a first-class compartment.

The businessman is traveling with his wife and her mother. He is escorting his mother-in-law, who had been staying for several months in his house in Lusaka to her village near the Tanzanian border. After unloading his mother-in-law, he plans to spend a week in his home village, which is also near the border.

"My wife all the time invites her relatives to come stay in my house. But I don't like a lot of relatives in my house. I live like a modern man," said Kakumgu.

Village-born and city-educated, Kakumgu travels back and forth on the train four or five times a year. Like many successful urban Africans, he says he loves and hates his village.

"I have that need for my village. If I did not go home I would feel guilty. But I soon get bored, and I always bring my own drinking water. My children -- I have five, the oldest is 15 -- have been to the village only once. They don't ever want to go again. They want lights and television. And the village water, they won't drink it."

In the late afternoon of the first day, having left behind the flat, empty, untilled Zambian savanna, the train labors on a not-very-steep hill. It slows from 25 mph to walking speed and then stops dead.

According to a Swedish mechanical engineer working on the rehabilitation project, the Tazara's track compares well with the most modern and best engineered in the world. With 320 bridges and 22 tunnels, it runs through some of Africa's most mountainous terrain. Most of the track can accommodate trains traveling up to 70 mph.

The problem has always been locomotives. The 2,000-horsepower locomotives supplied by China in 1975 proved gutless and unreliable. It took two Chinese locomotives to pull a train, especially one full of copper ore, uphill.

Some of the Chinese locomotives have been rebuilt with German engines, and 14 German locomotives have been added to the fleet. But, on average, only about 60 percent of the railroad's 80 locomotives work at any one time. If South Africa were to shut its transport corridors to the front-line states, the Tazara's current stock of locomotives could not come close to handling the traffic, railroad officials say.

That's where the Americans come in. The U.S. Agency for International Development has tentatively planned to supply 14 General Electric diesel locomotives.

These would mean that locomotive mechanics on the line would have to be familiar with repair manuals printed in Chinese, German and English. Repair shops would need large inventories of spare parts from each country. European railroad experts say they fear that the Tazara, dogged since its inception by spare-part shortages and bad maintenance, could become a Tower of Babel.

Standwell Mapara, general manager of the line, insists that proper training will eliminate any such problems.

In a second-class compartment sits Andrew Wala, a black South African carpenter with deep-set eyes, a nervous manner and no passport.

He says he is a "nonaligned" escapee from apartheid. He fled Cape Town in 1985, he says, and he was not welcome in Botswana, Zaire or Zambia.

"I was harassed," he said. "They told me to move on. In Zambia, the immigration people told me there was no asylum unless I joined the ANC." The African National Congress, based in Zambia, is the principal opposition movement to white-minority government in South Africa.

"I am not interested in the struggle," Wala said. "I want to get on top of my carpentry."

"I thought things would go in my favor when I left South Africa. But these {black-ruled} countries are not happy with my presence. I don't know what I will do in Tanzania. I don't know anything that will happen when I get there."

Late Saturday morning, when the train stopped near the Zambia-Tanzania border, Wala jumped out of the train. While border officials stamped passports and checked visas, he ran east for a mile or so and crossed the border. In the Tanzanian town of Tunduma, he sneaked back aboard and returned to his compartment.

It is cold in the Tanzanian highlands. The wind is strong and biting, bending back banana plants, rustling eucalyptus trees and rattling brittle cornstalks that have been left standing to keep farmland from blowing away.

As the train climbs toward the highland town of Mbeya (altitude 5,900 feet), clouds are at eye-level, blanketing the nearby mountain tops. Rivers run fast up here, houses are made of brick, not mud, and people wear coats.

Mbeya is the market center of the highlands, and the Tazara has made it a boom town in the past decade. Factories making soap, paper, cement and farm implements have opened. Farmers now have markets for their fruit, grain and meat. Neighboring Malawi trucks ferry produce to Mbeya for rail shipment to the port at Dar es Salaam. If South Africa shuts off southern ports, this town will become a key transport hub for the front-line states.

The loading platform swarms with people bound for the coast. Among them are four Chinese men in Mao suits. They are mechanical engineers, and they board the train.

"Are you Rooskie?" asks Chui Jiachong, seeing a white man in a corridor of the train. Upon hearing that the white man is an American, Chui seems disappointed. In his halting English, he says he studied Russian in Peking and had been looking for someone to practice it on.

There aren't many Russians to practice on in Zambia or Tanzania. Nor, for that matter, are there many Chinese. In the early 1970s, Chinese construction of the Tazara triggered a "Reds against the West" view of African development. Newspapers in Britain speculated that, in their spare time, Chinese railroad workers were training Marxist cadre. White-ruled Rhodesia and South Africa made much of the communist threat.

"The Tan-Zam Railway will be a Trojan horse for Chinese trade and a conveyor belt for the 'Thoughts of Chairman Mao,' " said Rowan Cronje, a member of parliament in Rhodesia.

Cronje was wrong. More than 15,000 Chinese came to Africa to build the railroad. Peking loaned Tanzania and Zambia more than $500 million for the project, interest-free. It was China's largest aid project. But when the railroad was completed in 1976, almost all of the Chinese went home. The loan remains unpaid and is probably unpayable. Tanzania and Zambia are not devotees of Chinese communism. Indeed, they are less socialistic than they were before the railroad.

Chui, the Chinese engineer, explained that his government no longer can afford to give large loans to the Tazara, only technical assistance. For that, there are about 100 Chinese railroad experts in Zambia and Tanzania. Money for locomotives and rail maintenance, Chui said, has to come from the West.

Eva Makunda and her year-old son, Yusufu, board the train at 3 a.m. Sunday. They are exhausted, having waited six hours for it. Owing to the Chinese locomotive, it is running late.

There is no room for Makunda and the child in third class. A security guard lets them sit on the iron floor outside the second-class dining compartment.

Makunda, a Swahili teacher who works in the central Tanzanian village of Chita, is on her way home to see her parents. She paid the equivalent of $5 to travel about 400 miles. She has no complaints about the seating arrangement. She at least can sit down to nurse her son. Back in third class, many mothers with babies must stand.

Forty-six hours after leaving central Zambia and four hours outside Dar es Salaam, a Tanzanian police officer observed this American reporter interviewing Makunda. He ordered me to follow him to his compartment. There, three plainclothesmen seized my notes and passport and asked when I had last been in South Africa.

In Dar es Salaam, security men escorted me and a West German journalist to a police station for three hours of interrogation. They also had discovered and arrested Wala, the South African carpenter with no passport.

Commandos from white-ruled Rhodesia blew up three bridges on the Tazara line inside Tanzania in 1979. The explosions closed the railroad for three months. The line lost money that year.

Growing tension between the front-line states and South Africa, along with recent cross-border raids by South African commandos in Zambia, Botswana and Mozambique, has raised fears that the Tazara may again be a target.

White faces on the train are viewed with suspicion. Armed soldiers guard every large station along the line. People who live near the track have been told to report any unusual behavior. There are at least three plainclothesmen on every passenger train.

"Our experience has taught us we should not take things for granted," said Mapara, general manager of the railroad.

After security officials had had two days to check the reporter's credentials and scrutinize notes, Mapara said, as if in apology, "Once bitten, twice shy."