The Beast: Riding the Rails and Dodging Narcos on the Migrant Trail. By Oscar Martínez. Translated by Daniela Maria Ugaz and John Washington. Verso; 267 pages; $26.95 and £14.99. Buy from Amazon.com, Amazon.co.uk

DO NOT be fooled by the strange jollity of the subtitle of this book. The world that Oscar Martínez, a Salvadoran journalist, set out to report on five years ago is so violent, depraved and hellish, you can hardly believe he survived to tell the tale.

At one end of his journey, in Central America, men and women are executed by gangs for reasons no one can understand, forcing those around them to flee for their lives to the one place they think they can be safe, the United States. At the other end, these hapless Guatemalans, Hondurans and Salvadorans, after weeks or months evading drug gangs and bandits across Mexico, float up “swollen, soft and pale”, on the banks of the Rio Grande. Even their faith in God—the only thing they had left—was unable to stop them from drowning at the border.

Along the way, migrants are shot dead and thrown off railway boxcars after whatever tiny bundles of money they have hidden upon themselves are discovered and stolen by well-armed gangsters. There are “bra trees” in the desert on the way to California where migrant women’s underwear is hung as trophies by bandits who have raped them in the wilderness.

What makes this courageous book extraordinary is that Mr Martínez sees all this first-hand. He takes eight trips huddled with migrants on the roof of La Bestia (“the beast” of the title), the train that takes them from southern to northern Mexico on the way to America, threatening at any moment to grind them with its steel wheels if they lose their frozen grip on the handholds. He sits among them during hold-ups, and feels the hooded eyes of narco scouts on every step of his journey. He even wades the treacherous Rio Grande to see if there is a safe route across.

But this is not a book about him. In fact, he rarely reveals his personal feelings, except through rugged prose, beautifully translated by Daniela Maria Ugaz and John Washington, that lets the injustice speak for itself. Instead the story of “The Beast” is told from the viewpoint of the migrants “clinging like ticks” to the train’s roof-struts. “A migrant passing through Mexico is like a wounded cat slinking through a dog kennel: he wants to get out as quickly and quietly as he can,” he writes.

Their stories are as vivid as they are shocking; the senseless violence pervading their lives evokes another classic book about Central Americans, but from a different era, Joan Didion’s “Salvador” (1983). “The Beast” is also a work of journalism (Mr Martínez is an editor at a prize-winning online news channel, ElFaro.net). He weaves in well-sourced statistics on the size of the migrant problem. He pillories Mexican authorities for turning a blind eye (or worse) to the problem. And he deftly depicts the stupidity of much of America’s immigration policy, while explaining its history and showing some sympathy for the cat-and-mouse game played with migrants by border-patrol agents.

Two chilling insights stand out, as Mr Martínez, after abandoning La Bestia, wanders from one side of Mexico’s bloodstained border with the United States to the other. The more America seals off the frontier, the more migrants are forced into a race with drug “mules” for the remaining gaps. That amplifies the violence. Migrants are powerless to report the cruelties they suffer; no authorities want anything to do with them.

Thankfully, some priests along the route offer them food and shelter. The reader longs to hear more about these good Samaritans because they are as rare as hens’ teeth. The journey through Mexico’s backwaters that these migrants take is more lawless and brutal than anything Graham Greene described in the 1930s. So awful, in fact, that after reading “The Beast” you cannot help but conclude that most of them make it not out of choice, but out of desperate need.