The train jerks and halts with little notice -- at least not for the passengers stowed away on top of it. There are hundreds of them, or perhaps thousands, and as the train bucks, they search desperately for a safe hold among the yards of smooth steel. It's not meant for passengers up atop the train, but where they're headed, no seats are offered, and they have already paid dearly for this space.

You see, America is a cruel mistress, and one must take any in there is to cross into her borders. For Central American immigrants passing through Mexico, that means climbing aboard the Death Train.

La Bestia, or "El Tren de la Muerte," as it's also known, is the easiest way for Central American immigrants illegally crossing through Mexico to get to America. On ground, they risk police checkpoints and violent gangs, and the brutality can be much worse. So they cling to hope -- and steel -- across the Death Train, which they board illegally in south Mexico, paying off the armed gangs that guard them.

But that payment only satisfies the toll for the guards; the payment to La Bestia is often costs much, much more.

As the train whips along the rickety tracks through Mexico, the Guatemalans and Hondurans, and even the Salvadorians who have boarded atop the train illegally cling as close to the train's surface as they can. It's slippery along the slick surfaces of the train, and the lucky ones hold tight enough to evade falling under the whirring wheels below.

The ones that sleep or lose their grip? Well, they aren't always as lucky. 

Hundreds of thousands of illegal immigrants pass through Mexico from Central or South America each year. While some will take up residence in Mexico, working temporarily and migrating from place to place, most are headed for what they call El Norte. And most of them opt for the Death Train.

The Mexican freight train in which these Central American immigrants are hitching a ride makes a point to live up to its name. While some will make it to the American border unscathed, many others will not. A handrail is the only protective barrier between the train's top and the tracks, and one false move could prove fatal. It does prove fatal, often.  

An errant tree branch, an unexpected cable; the deadly obstacles often appear without warning, and take another life or limb from the train. It's the nightmarish price of riding the Death Train to freedom, and thousands will still take the gamble, despite the risk.

While physical obstacles may be the biggest threat aboard the Beast, the threats from Mexican cartels or gangs who board the train and rob passengers are just as deadly. Even the Mexican police are known to demand a toll. The threat of alerting immigration often gets them what they're looking for. 

Each day, La Bestia takes lives as payment for her services. If the passengers are lucky enough to make it, they arrive damaged and torn. In order to survive the Beast, one must not fall asleep. One must not slip. And one must be lucky enough to survive derailment, should that happen.

And happen it does. In August 2013, La Bestia crashed in the Mexican state of Tabasco. The derailment killed 11 Central American immigrants, and injured 18 more. Train derailments are common for La Bestia, as are the assaults, rapes and murder that the passengers face.

Shelters in border towns have been put in place for the unaccompanied children who arrive from Central America, and are run by private groups like Catholic Charities in order to combat the overcrowding and horrific conditions of youth immigrant detention centers. The conditions of these facilities were the subject of recent news reports in which journalists reported overcrowding and filthy living conditions after being given access to the centers.

The influx of illegal immigrants from Central America has been deemed a "humanitarian crisis," with many of the migrants being unaccompanied children. 

Immigration reform has become a hot-button issue as debates among politicians ramp up in anticipation of the 2014 elections. Both Hillary Clinton and Joe Biden have stated in recent interviews that they believe undocumented minor children should be returned to their home countries.

For the passengers on La Bestia, that means making it off the train alive first.