The other border

Central American migrants sit atop a freight train headed north, as lightning flashes across the sky in Arriaga, Mexico, early on Aug. 4, 2013.

Thousands of immigrants ride the trains, known as "la bestia," or the beast, during their long and perilous journey through Mexico to the U.S. border.

Many of the immigrants are robbed or assaulted by gangs who control the train tops, while others fall asleep and tumble down, losing limbs or perishing under the wheels of the trains. Only a fraction of the immigrants who start the journey will arrive safely on their first attempt to illegally enter the United States.

An Indigenous family walks from Guatemala into Mexico after illegally crossing the border at the Suchiate River in Talisman, Mexico, Aug. 1, 2013.

They passed directly under a bridge with a Mexican immigration checkpoint. Thousands of Central Americans pass illegally into Mexico daily, many on the first leg of their long and perilous journey north towards the United States.

A group of undocumented Guatemalans is ferried accross the Suchiate River into Mexico, Aug. 1, 2013.

Raft operator Maximiliano Ortiz, 16, waits for immigrants to ferry accross the Suchiate River from Guatemala into Mexico, Aug. 1, 2013.

He passes his raft directly under an international bridge with a Mexican immigration checkpoint.

Guatemalan laborers carry products from Guatemala into Mexico to sell, in Talisman, Mexico, Aug. 1, 2013.

Illegal commerce passes back and forth at the border crossing.

Undocumented Guatemalans disembark after crossing the Suchiate River from Guatemala into Mexico, Aug. 1, 2013.

Undocumented Guatemalans carry their belongings after crossing the Guatemalan border into Mexico at the Suchiate River, Aug. 1, 2013.

A laborer carries products from Guatemala into Mexico in Talisman, Mexico, Aug. 1, 2013.

Jorge Hernandez from Guatemala sells candy and cigarettes on the Mexican side of the Suchiate River, Aug. 2, 2013 in Ciudad Hidalgo, Chiapas, Mexico.

People cross the Suchiate River on rafts from Guatemala into Mexico in Ciudad Hidalgo, Chiapas, Mexico, Aug. 2, 2013.

Merchandise is loaded onto a raft to cross the Suchiate River from Mexico into Guatemala, Aug. 2, 2013.

Commerce passes each way on the river, to avoid customs duties.

Central American immigrants wear plastic bags during a thunderstorm while atop a freight train about to head north, early on Aug. 4, 2013, in Arriaga, Mexico.

Central American immigrants check a map of Mexico before boarding a freight train headed north, Aug. 3, 2013 in Arriaga, Mexico.

Thousands of migrants ride atop the trains during their long and perilous journey through Mexico to the U.S. border.

Central American migrants climb atop a freight train headed north early on Aug. 4, 2013 in Arriaga, Mexico.

A train crew loads sand onto an engine to sprinkle on rain-slick tracks while headed north early on Aug. 4, 2013 in Arriaga, Mexico.

Central American immigrants prepare to board a freight train headed north in Arriaga, Mexico, Aug. 3, 2013.

Central American immigrants sit atop a freight train about to head north early on Aug. 4, 2013 in Arriaga, Mexico.

Migrants line up for lunch at the Hermanos en el Camino (Brothers in the Road) shelter in Ixtepec, Mexico, Aug. 4, 2013.

The shelter, founded in 2007 by Catholic Father Alejandro Solalinde Guerra, houses and feeds immigants, most from Central America, during a stop on their train route through Mexico towards the U.S. border.

Immigrants arrive at a rest stop after a 15-hour ride atop a freight train headed north early on Aug. 4, 2013 in Ixtepec, Mexico.

Immigrants arrive at a rest stop after a 15-hour ride atop a freight train headed north early on Aug. 4, 2013 in Ixtepec, Mexico.

Thousands of migrants ride atop the trains, known as "la bestia," during their long and perilous journey through Mexico to the U.S. border.