By Duncan Kennedy
BBC News, on the Mexican-Guatemalan border
Makeshift rafts are used to get across the river border
As the debate over immigration policies continues in the United States, the number of illegal immigrants heading to the US shows little sign of decreasing.
Time-lag politics is being exploited by real-time action.
We began looking for evidence down on the Guatemalan-Mexican border.
It all starts with rafts - two inner tubes lashed together with a small wooden plank on top. Dozens of them are pushed every day across the river separating the two countries.
Illegal migrants on the rafts are easy to spot.
They are the ones with the small backpacks, showing that on journeys like this the personal gives way to the practical.
One of them, 19-year-old Rosa, could not even afford a backpack.
So she left home with a plastic bin liner containing a spare pair of jeans and some toilet paper. Nothing more - that is all she has to make the long trip to the American border.
Mid-river, Rosa tells me: "I'm not frightened of making the journey alone."
But her eyes betray an unconvincing sincerity.
"In America I can live for a while, return and give my son a better life," she says, referring to her two-year-old child who she has left behind in Guatemala.
On the Mexican side, we wave Rosa off. Her plan is to use cars and trucks to get to the US border but most migrants prefer freight trains.
We head for a town we have been told of, called Tenosique, in southern Mexico.
Tonight, last night and tomorrow night it is the same scene. In a field by the railway tracks, hundreds of illegal migrants wait for one of the rare north-bound freight trains.
With sat-nav precision they all arrive, by foot, in the same field every day.
We talk to some of those who have been waiting - and waiting.
"Four days," says Leidi, sitting on the hot tracks. What has it been like?
Jenny was willing to jump on the moving train with her baby
"Awful," she says.
She tells me she is a doctor in Honduras.
"A doctor who's turned into an illegal migrant?" I ask, struggling to contain my disbelief.
"Yes," she says, " I have no choice. I cannot earn a good living in Honduras."
With professional classes on the move, Honduras is being drained of the very people it needs to help it out of its poverty.
In the same field we find a 12-month-old baby girl, Maria. Her mother, Jenny, says, "The food is better in America, so is the education and the jobs. I have to give my daughter a chance."
"Even it if means jumping onto a moving train with your baby?", I ask.
"Yes," she says.
It is not until late into the night the train finally rumbles out of the darkness.
The migrants get to their feet. Back packs are secured to backs. This train will not stop.
Migrants take huge risks in their bid to travel north
The chaos starts almost immediately. As the train thunders by at up to 50km (30 miles) an hour a few seize the chance.
They make a grab for the ladders attached to the sides of the train's wagons. Some get a grip and haul themselves on. Others falter, their fingers loosened by the sheer speed of the train.
Ahead, I can see people stumble in the darkness. The ground is uneven and there are other migrants in the way of a clear run.
Some turn their backs to the train. The futility, the danger, too great.
Limbs have been lost in ventures like this. Lives, too.
It is crazy but it is also compelling. Fail to do this and the migrants' dreams of reaching America effectively end in this desolate field.
After the five minutes it takes for this long train to pass, its tail lights finally come into view.
For the scores left in the field, it is a frustrating time. Another long day in the sun awaits.
With the help of a local driver we chase after the train. Eventually, we find it as it slows to a point where we can get on. Even at this pace it is hard to clamber up the metal ladder to the top.
The train clangs back and forth as it sets off again. Thinking I was going to be thrown off, I was petrified before its wheels had made a single revolution.
Once I had secured my grip I looked around. The sight defied belief.
Every space on every wagon in front and behind was taken up with illegal migrants holding onto roofs.
Some migrants bring weapons to defend themselves against robbers
Alven became my travel companion and told me why he was trying to get to the US.
"In Honduras, I earn $2 a day. In Los Angeles, $10 an hour.
"That's why if the Americans build a wall it won't work. People will still keep coming to make a better life for themselves, " he says.
It is human nature, he tells me, for people to want to improve their lives.
It does not feel very human, or natural, clinging to the top of my freight wagon. You have to fight tiredness. You never know when the train is going to stop.
At a curve in the track, the train slows and we get off. The migrants continue. Many will not make it.
If they get over the Mexican-US border, they could well be arrested in the deserts of Arizona and Texas.
But that will not stop others coming in search of what they see as their more promising land.