From NetFlix:

Fleeing retaliation from the violent Central American street gang he has deserted, young hood Casper (Édgar Flores) boards a northbound train, where he takes refuge on top of the moving freight cars and hopes for a fresh start in a new country. Dodging authorities and other dangers, he finds a new friend in Sayra (Paulina Gaitan), a Honduran girl also making a run for the American border. Cary Fukunaga directs this foreign-language thriller.

In Spanish with subtitles, this grim tale of the desperate desire to escape a hopeless life in the poor, gang-ruled parts of Mexico makes for some difficult watching. You can read about the plot in the
Wikipedia article. I had hoped that article would confirm that the film is an accurate portrayal of life in parts of Mexico. Unfortunately the only quote of note from this article was the following:

The film was shot in Torreón, Coahuila, Mexico. Several of the extras used in the film were actual migrants. Fukunaga said of working with them, “I didn’t have to tell them anything – they know how to sit on top of a train.”

Someone once defined “morality” for me as “that which you must do to receive the approbation of those around you”. Keep this in mind while you watch the behavior of the gang members.