LIKE A THIRD-CLASS COACH on a rickety railroad, Paul Theroux's new travel book offers fleeting glimpses of scenic beauty, even more fleeting glances into other people's lives and long stretches of discomfort, fatigue and tiresome companions before leaving you, finally, at a cold dark station in South Godforsaken doubting the trip was worth it.

Theoroux is a busy and restless writer much of whose fiction is set in far places. But his best-known book, The Great Railway Bazaar, is a nonfiction account of his four-month train journey through Europe and Asia. It is rich with the romance of the world's famous trains -- the Orient Express, the Japanese bullet trains, the Khyber Mail, India's Grand Trunk Express, the Trans-Siberian. The music of those names mutes Theroux's sometimes cranky complaints about the people and countries he saw from their windows. And anyway, a few complaints could be forgiven someone involved in such an endurance test. "I felt flayed," Theroux wrote at the end. "It was as if I had undergone some harrowing cure, sickening myself on my addiction in order to be free of it."

The cure didn't take. Two winters ago, Theroux was off again, obsessed with his observation that there was an almost continuous line of track from the subway station near his childhood home in Boston to the town of Esquel in southern Argentina. One cold morning he boarded the subway. "I craved a little risk," he explains, "some danger, an untoward event, a vivid discomfort, an experience of my own company, and in a modest way the romance of solitude." But there is very little romance in The Old Patagonian Express.

Theroux recounts his experiences on every train along his route, first to Chicago, then down to Texas, through Mexico and Central America, broken by a plane trip from Panama to Columbia, and then along western South America through the Andean highlands of Ecuador, Peru and Bolivia, and finally across the mountains and pampas to Argentina.

The trains, he discovers, are different from those he rode in Europe and Asia. The geography and economic history of Latin America produced railroads designed more for shipping raw materials and wealth out of the various countries that for the comfortable transport of well-to-do passengers. "Only the semidestitute, the limpers, the barefoot ones, the Indians, and the half-cracked yokels took the train, or knew anything about them," Theroux writes. "For this reason it was a good introduction to the social miseries and scenic splendors of the continent."

He sees plenty of social misery, beggars, poor Indians, homeless waifs on the streets of wealthy cities. In Bogota, Colombia, struck by the numbers of children begging, he ponders an observation from Boswell: "A decent provision for the poor is the true test of civilization."

He sees scenic splendors as well, but does not dwell long on them. What he does lavish detail on are his discomforts. The crankiness of Railway Bazaar here becomes a continent-long complaint. Theroux describes his altitude sickness and his stomach cramps, his aching teeth and the hand he cut on glass in a hotel sink. The trains are dirty and thieves may be after his watch. His fellow passengers, almost without exception, drive him to distraction with their inane conversation.

The problem is not just that his tone is irritating. It is that he is so firmly convinced his commonplace complaints are of compelling interest. Theroux is certainly capable of writing well about travel. He has, for example, a sure sense of that secret joy of starting a long trip early in the morning, alone, while others begin their daily routine. "The poetry of departures," he calls it. He knows that airplane travel, with is relentlessly cheerful flight attendants, its X-ray machines, its controlled climate and pressure, its wrenching vaults across time zones offers not adventure but only apprehension; that the magic of travel is best felt on the earth's surface where landmarks appear on the horizon, march ever more quickly towards you, then whip suddenly into the past.

But Theroux so loses himself in the mechanics of how he got to Patagonia, and the people who irritated him along the way, that there is little room in the book for anything else. And since not very much out of the ordinary happened to him, one's interest flags.

Nor is it much revived by his observations on the people among whom he passes. There are a few sympathetic and enjoyable conversations, mainly about English and American literature, with the blind Argentine writer, Jorge Luis Borges, in Buenos Aires. There are Theroux's comments on the insularity of the Americans in the Panama Canal Zone during the time when controversy over the new treaty was raging. But there is little else to recommend the book. Theroux's goal was not very ambitious -- simply to reach the end of the line. He was, in his own words, "merely skimming south, a bird of passage generalizing on the immediate." He reached Esquel and got off the train. Was he wiser? He now knew what Patagonia looked like. And what had he learned about the countries through which he'd skimmed? "Latin Americans are funny," an American embassy political officer told him in Ecuador. "They hate to be criticized. They can't take it." That Theroux found this man impressive suggests the answer.