The Ferrocarril del Istmo de Tehuantepec (Ferrocarril del Istmo de Tehuantepec, S.A. de C.V.), also known as Tren Interoceánico, Line Z, Ferrocarril Transístmico or simply Ferroistmo, is part of the Interoceanic Corridor of the Isthmus of Tehuantepec, owned by the Mexican government, that crosses the Isthmus of Tehuantepec between Puerto Mexico, Veracruz, and Salina Cruz, Oaxaca. It is leased to Ferrocarril del Sureste FERROSUR.

The Interoceanic Corridor of the Isthmus of Tehuantepec, abbreviated as CIIT, is a trade and transit route in Southern Mexico, under the control of the Mexican Secretariat of the Navy, which connects the Pacific and Atlantic Oceans through a railway system, the Ferrocarril del Istmo de Tehuantepec, for both cargo and passengers, crossing through the Isthmus of Tehuantepec. This project also consists on the modernization and growth of local seaports, particularly the ports of Salina Cruz (Oaxaca) and Coatzacoalcos (Veracruz), and of the Minatitlán oil refinery and the Salina Cruz oil refinery. In addition, it plans to attract private investors through the creation of 10 industrial parks in the Isthmus area, as well as two other parks in Chiapas. The project has the goal of developing the economy and industry of the Mexican South through encouraging economic investment, both national and international, and facilitate commerce and transportation of goods internationally.