The blast of the train whistle breaks the tedium of the night, and the lumbering 100-car freight slowly begins its journey north. "That whistle means get on if you're going to go," says Willie Castleberry, and along the tracks, the bushes come alive with men rushing for the cars.

It is 9:39 p.m. Wednesday, a mile or so north of the Rio Grande River. Castleberry and H. G. Pool, members of the Border Patrol, have been here for more than an hour. They've been deploying their limited forces for a roundup of undocumented Mexicans who have punctured the leaky U.S.-Mexican border and are trying to escape north for a job that could take them out of poverty.

This tide of illegal immigration is what President Reagan hopes to shut off with his new immigration policy, but it is impossible to believe that any government program, short of massive force, will stem the flow of people willing to risk their lives for a chance at the American dream.

The agents know it and the Mexicans know it; what goes on here at night is a ritual in search of a policy.

"It's like hide-and-seek," Castleberry says, "except we're all grown-ups. We tage the wets [for "wetbacks"] and take them back to the border and let them go. And they come right back."

This is a dangerous place in the darkness. A misstep can send a man sprawling along the gravel roadbed and under the moving train, whose steel wheels can sever a human body in an instant.

Castleberry, who carries a chew of tobacco in his jaw, is in charge of the 4 p.m.-to-midnight shift, and his agents come here four or five nights a week. His men have asked railroad officials to move the train out of the yard slowly to make it easier to pull off the Mexicans as it begins to roll, but even at one car every five seconds, it is a risky business.

Flashlights flicker along the train like low-wattage strobe lights, as the agents look into box cars and under the wheels of the truck trailers stacked on the piggyback cars. The agent's two-way radios crackle up and down the tracks.

Suddently faces appear in the beam of light, eyes frozen with fear. The Mexicans race down the flatcar, with agent Bruce Simonds running behind them. The Mexicans disappear into the darkness. In a few minutes they are all back, two in handcuffs. In another hour, the two will be back in Mexico, ready to try again.

Last year, Border Patrol agents captured 38,681 people along the 100-mile border that falls under the Laredo office. Along the entire 1,900-mile U.S.-Mexican border, 742,557 apprehensions were made. For each one captured, one or two got through. Many of those captured were repeaters.

Francisco Reyes Penaloza is one who has made the illegal trip many times. He is in the Webb County Detention Center a few miles north of Laredo. He was captured on June 16 about 60 miles to the north, after slipping past Immigration and Naturalization Service agents at the International Bridge in Laredo with the help of a smuggler, a man whom he paid $200 for a phony birth certificate and a ride north.

Penaloza says he wants to attend a U.S. college but cannot afford the cost of a student visa and the bribe he says he would have to pay Mexican officials to get proper papers. He says he was working at a factory in Denver and hoped to learn industrial design. In the United States, he says, "It is easier to come and study and be able to work at the same time."

He went back to Mexico to visit his mother, who he says was ill. He stayed 2 1.2 months before returning. Now he is disillusioned.

"Being here for four months [in the detention center], I'm wasting time," he says. "I will lose by job. I don't like the feeling of being detained. It demoralizes me."

The freight train's red caboose clips past Castleberry's command post at 9:55 p.m. The roundup has netted 24 men. "There weren't many who got out on that one," Pool says.

But in the more than 90 minutes that the agents were waiting along the tracks, other Mexicans were slipping across the unguarded Rio Grande and beginning their trek north through the rattlesnake-infested brush country. There aren't enough agents to cover both areas at once.

Back at the Border Patrol headquarters, the 24 Mexicans, who vary in age from 17 to 48, are quickly processed by the agents, and at 10:30, the men were loaded into a van for the return trip to the border. It is all done quickly and with occasional humor. Tonight thee is no trouble.

After all, there is a morning train to catch in Laredo.