Rail travel is not about to boom South of the Border. It's even doubtful if Amtrak--under whose aegis Theroux begins this new, mostly dispiriting journey--will pick up an extra fare. But, unforeseeably, the point is otherwise. The opening sings: Theroux wakes up in the Medford, Mass., bedroom of his childhood and catches the train--the workaday commuter train--for Patagonia! But on the Lake Shore Limited he encounters only a raw-food freak and a fiftyish ingenue worth mentioning; and heading South, only the ""suffocating"" towns of Oklahoma and Texas. Crossing from Laredo to Nueva Laredo is merely to swap banal order for tatty squalor--and board the first of the world's most run-down, slow-moving, altogether hopeless trains. No Latin American who has a choice, Theroux discovers, travels by train; his companions are the noisy, uncommunicative poor--and, memorably, a garrulous, forlorn American out for a good time. He reads, or tries to. The local to Tapachula, so long stalled at one point that it ""had become a part of the town,"" becomes the place, too, that he dreads finishing Pudd'nhead Wilson; in a San Salvador hotel he begins--until visited by a rat--Poe's terrifying Narrative of Arthur Gordon Pym. Later he'll take up Boswell, and interleave excerpts. And somewhere along in this futile, quixotic quest, the traveler takes precedence over the travel. True, Theroux's distress at the desolate landscapes, the decrepit hamlets and urban sinkholes, the vacant, brutalized people is occasionally relieved; parts of Costa Rica, Ecuador, Argentina have, for him, their qualified charms. But it is he who compels interest: passing an idle afternoon flossing his teeth; parlaying a wounded hand into fast service and bargain prices; free-associating to the clack of the wheels. Train travel, he maintains, exposes ""the social miseries and scenic splendors of the continent""; but these are familiar views-in-passing. What we realize, from his remark that few trains cross borders, is how fragmented the continent is; and the paucity of long-distance travelers reminds us that it's also a cul-de-sac. So it's fitting that Theroux, after lingering in Buenos Aires to play Boswell to Borges (a lambent episode), ultimately finds himself alone at a Patagonian railway junction at 2 A.M. menaced by a pack of rabid (?) dogs; in his mind, facing death. Boredom, loneliness, sickness, fear--these, met and faced down, are Theroux' trophies. With almost none of the popular appeal (including the erotica) of The Great Railway Bazaar, this does have the staying power of a slow bleed.