The Isthmus of Tehuantepec Interoceanic Corridor (CIIT) comprises a 308km railway line between renovated ports at Salina Cruz in Oaxaca state and Coatzacoalcos in Veracruz.
Service is expected to start in December with two daily round trips for passengers, and three for cargo.
The project seeks to capitalize on the periods of low water levels affecting the Panama Canal as the region suffers increasingly frequent droughts.
For President Andrés Manuel López Obrador, it forms part of a gambit to draw investment to the poorer south.
