Mexican President Andrés Manuel López Obrador has launched the Maya Train - an ambitious project to build a railway through five south-eastern states.
At an indigenous ceremony near the ancient Mayan ruins of Palenque, he said this would help create hundreds of thousands of jobs.
The 1,525km (950-mile) railway will link Palenque and the tourist resort of Cancún on the Caribbean coast.
But opponents warn it could damage the region's important biosphere, reports the BBC.
A third of the planned route passes through pristine jungle, and the owners of the land, the Maya people, say they have not been consulted.
But President López Obrador dismissed environmentalists' concerns, promising that "we won't uproot a single tree".
The four-year project is expected to cost up to $7.4bn (£5.9bn).