People could be seen crammed into a freight compartment, in a photo published by the Televisa channel on Sunday. There were some 200 Central Americans in each truck, including children.
Thousands of migrants have marched to the Mexican border with the United States in the past couple of years in hope of getting asylum, prompting President Donald Trump to declare a national emergency.
Trump accuses Mexico of not doing enough to stop waves of migrants and has threatened the neighbouring country with steadily rising import tariffs if arrivals did not stop. Last week, Mexico promised to send 6,000 troops from its National Guard forces to the border with Guatemala to fight human smuggling networks. The US, for its part, said it would send asylum seekers who attempt to cross the border back to Mexico, where they will wait for their cases to be addressed.