That is the main rail line, known as The Beast, that runs all the way from Guatemala to Texas, all the way North. And yes, they’ve been allowing it as long as they stay on the tops of the cars. I suppose they get a hot and a cot and a shower and a new set of clothes somewhere along the way, certainly when they get to the end.
Thanks! I looked around and no one has what railroads own the trains, just that it's a network of Mexican freight trains which transport products and materials including corn, cement, and minerals and are climbed on and ridden by migrants illegally.
So, there are cars owned by other companies than CMEX on the trains, and it's really the reponsibility of the railroads that the migrants are riding them, not the individual car owners.