The town of Chahuites, Oaxaca, is a sleepy little village surrounded by mango farms. A line of train tracks cuts through the southern edge of town.

Chahuites is in an isolated part of southern Oaxaca, about 170 miles north of the Guatemalan border. Migrants from Central America used to just pass through town riding on top of La Bestia, the train migrants traditionally traveled on across Mexico. But now immigration agents patrol the train, forcing migrants to walk northward along the railroad tracks. 

“People wait by the railroad with machetes and guns to rob migrants,” said Juan Vicente, a migrant from El Salvador who works at a construction site in Chahuites, “then they steal whatever you got.”

It takes some migrants weeks of walking just to reach Chahuites. Migrants now spend more and more time in southern Mexico, in part because of an immigration enforcement strategy called the Southern Border Program. Mexico launched the program back in 2014 when thousands of Central American families and children fleeing danger back home arrived at the U.S.-Mexico border. Washington called their arrival a “surge” and pressured Mexico to stop the flow of migrants. Mexico created the Southern Border Program in response, an initiative supported with millions of dollars in U.S. funding, that sent more immigration agents to southern Mexico, increased surveillance of trains and built new highway checkpoints.

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La Bestia, once loaded with migrants huddled on top of boxcars, is now largely empty when it passes through Chahuites.

In the years since the Southern…