This kind of encounter is further encouraged by the tendency of Amtrak conductors to seat long-distance passengers next to each other, even if the next car contains 20 rows of empty seats. This policy is designed to keep rows open for passengers who board at later stops, but sometimes those anticipated passengers never materialize. The two communal cars also tend to encourage interaction: the Sightseer Lounge has open seating and is especially busy after lunch, and in the dining car, the host will combine any groups smaller than four people with strangers in order to fill every booth. Groups also coalesce at stations like Beaumont and El Paso in Texas and Tucson and Maricopa in Arizona, where the train stops for cigarette breaks. And at longer stops, like the nearly three-hour wait in San Antonio, passengers often venture out into the city together, heading to Denny’s for a midnight meal or to Alibis’ Sports and Spirits bar. Nobody on a long-distance train is ever really alone.

And so it was that Matthew Carr, who limped down the aisle to the seat that had been vacated by Suzy Lanza, found himself amid people in similar circumstances. Carr, 32, was a striking figure; a Marine veteran with strong shoulders, wary eyes and a sweep of whitish blond hair that fell across his forehead. Sitting in front of him were a 2-year-old boy, Sincere Prince Hernandez, and Sincere’s 28-year-old mother, Selena Hernandez, a large, attractive woman, six feet tall, with a round beaming smile and a royal blue kerchief tied around her head. Like Carr, Selena and Sincere were headed home, and like Carr, Selena knew that when she arrived, her home would no longer feel like home. Carr and Selena therefore belonged to the fourth, most fragile category of long-distance traveler: people who are starting over.

The previous week, Selena kicked her husband out of her house; she suspected that he was unfaithful and demanded a divorce. She feared that he would take their three children. He would stand outside on the street at odd hours, she said, watching the house. Selena decided to drop her older children with relatives and drove with Sincere and her grandmother to New Orleans to stay with family there. “Don’t mess with nobody from Belize,” said Selena, whose husband was Belizean. “For that matter no Haitians, no Panamanians, no Africans, no Dominicans. Puerto Ricans, you can work with.”

But soon after she arrived in New Orleans, Selena realized she couldn’t stay — her aunt had little patience for a 2-year-old’s antics. Then Selena’s grandmother, without warning, drove off with the car. There was nothing to do but take the train back to Los Angeles, even though Serena worried that her husband would be waiting for her. “My house might well be trashed,” she said, watching her son as he explored the darkened train car, racing up and down the aisle. Sincere, trusting and curious, was adopted by his fellow passengers, including Carr. He occupied himself for long stretches of time by serving as the train’s janitor, collecting gum wrappers, empty plastic cups and crumpled tinfoil and depositing them in the garbage basket.

At the San Antonio station, the train was connected to the newly arrived Texas Eagle, which originated in Chicago. Two passengers got off the train and went to a bar; when they returned, one was bleeding from the face. San Antonio Police arrived and escorted the men to jail.

After 2 a.m., the Rodriguez and Escamilla families boarded. There were eight of them — aunts, uncles, cousins and great-grandparents of a boy who was celebrating his 1st birthday in Phoenix. After the child’s party, they planned to head straight to Las Vegas to gamble. They chatted loudly, laughing, oblivious or indifferent to the fact that Sincere was writhing exhaustedly in his mother’s lap and Matthew Carr was trying to find the position in which he could sleep with minimal pain.

The Sunset Limited was inaugurated in 1894, but its name comes from a pre-Civil War route: a train that departed Harrisburg, Tex., at sunrise would arrive at the route’s terminus in Columbus, 80 miles away, at sunset. Today it takes two sunsets, and two sunrises, for the Sunset Limited to reach its destination — provided there aren’t delays. When the sun set in Houston, the train was still in the South; by sunrise it had unmistakably entered the West. The bayous and flooded forests of southern Louisiana were replaced by a vast, sandy expanse interrupted by the occasional exclamation of a yucca plant, its pinkish flowers rising from an asterisk of spiky leaves. Rough dirt roads led to stables with horses dappled the same colors as the landscape: dark brown, cream, chestnut. In the distance marched a succession of flat-topped, sloped and nippled buttes. A canoe lay overturned in the dirt next to the track. The Sunset Limited had been in Texas for 16 hours; the New Mexican border was still more than 6 hours away.