Mexico City, Mexico — Hundreds of Central American migrants trying to make their way to the United States have been stranded in Mexico in recent weeks after the freight-train line they ride — labeled "The Beast" — began enforcing a ban on stowaways, migrant advocates say.

The migrants, often desperate to leave poverty and crime in their home countries, brave riding atop the train cars, activists say, and are prey to criminals who have been accused of rape, robbery, and murder.

Many of the migrants have ended up in already crowded shelters, which have reported populations now swelling beyond capacity with men, women, and children. Migrant activists say that once the migrants are stranded, they face a difficult decision of whether to return home or try the journey north by foot or car, leaving them vulnerable to criminals along the way.

Crew members of the railroad normally turn a blind eye to migrants scrambling to stow away on the trains - a reality which is featured in books and documentaries about the migration north.

But after the southern Mexico state of Veracruz filed a lawsuit last month accusing the railroad of complicity in the violence on the trains, the train operators began banning the stowaways, migrant advocates said. The train line is run by Ferrosur and Kansas City Southern de Mexico.

According to activists, train cars overloaded with migrants have been unhitched and left idle, and the train has skipped stops where scores of migrants waited.

Another rail line known for carrying Central American migrants continues to allow migrants on board, according to human rights activists who advocate Mexico’s granting of travel permits so the migrants can freely head north in better conditions.

José Alberto Donis Rodríguez, manager of "Brothers on the Road," a migrant shelter in the southern state of Oaxaca, said recently that about a third of the approximately 1,000 migrants stranded in Oaxaca for over a week, decided to continue the journey on foot.

Fray Tomás González, head of "72 Migrants," a shelter in the state of Tabasco near the Guatemala border, said the number of migrants seeking shelter there has tripled, which he attributed both to the enforcement of the stowaway ban and growing numbers of migrants fleeing their homelands.

Officials at Ferrosur and Kansas City Southern de Mexico declined to say whether they had stepped up enforcement against those who hitch rides on the train. But in response to media queries, Ferrosur said in a news release that it has always been obliged to warn people of the "grave dangers" in boarding a freight train without authorization.

"All of our staff, including machinists, must respect the integrity of those who illegally travel on top of the railway cars," read the statement.

The journey has always been long and arduous, but Central Americans are increasingly taking the risk.

At the Rio Grande Valley in Texas, a popular crossing point for Central Americans, the Border Patrol made more than 90,700 apprehensions in the six months ending in April, a 69 percent increase over last year, and an increasing number of the migrants are making claims for asylum in the US.

Federal authorities in Mexico, which sporadically stage a crack down, last week arrested and prepared to deport nearly 300 of these migrants, most of them Hondurans, who were traveling by foot through the state of Tabasco.

The Rev. Alejandro Solalinde, a veteran migrants’ rights advocate in Mexico, accused the authorities of human rights violations during the raid, including verbal and physical aggression against women and children.

He said the migrants were quickly deported, without due process to allow them the possibility of applying for humanitarian visas.

Aurora Vega, a spokeswoman for Mexico’s immigration agency, referred to the detentions as a routine rescue operation conducted by "all legal means." She said she was unaware of any new effort to clear migrants from trains.

"It is an issue that does not concern us since it is private companies who operate the railways," she said.

The Beast stops south of the United States border, and migrants who make it that far then try to cross by river or through the desert.