A cargo train carrying 300 Mexican migrants to the United States derailed Sunday, killing at least six and injuring 35.
The freight car, well known for carrying migrants on its roof, slid off the track in the southeastern Mexican state of Tabasco.
It is believed 16 people are in serious condition. One of the dead is from Honduras.
Eight of the 12 freight cars were overturned and rescuers were on the scene to free those trapped under the train.
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The accident was said to have occurred far from roads or towns, making the rescue particularly difficult.
The train was only accessible by air or boat.
The derailment is now being blamed on missing track nails.
Tabasco state's Public Security Minister Audomaro Martinez Zapata said thieves had "ransacked" nails and metal plaques that should have been part of the tracks.
The so-called "Beast" is an infamous train that carries Mexican and Central American migrants from as far south as Guatemala to the north of Mexico.
The trip is full of danger for migrants from bandits to accidental injury and death.
AFP reported that about 140,000 migrants enter Mexico illegally each year with the intent of traveling on to the United States.