The Southern Pacific "Inter-Cal" line branched from the SP's Sunset Route at Niland, CA, ran southwards and entered Old Mexico at Calexico, CA/Mexicali, MX. It ran for some 50 miles east and re-entered the U.S./California at Algondones, MX, connecting with the SP Sunset Route at Araz Junction, about 5 miles west of Yuma, AZ. It was this railroad that the SP used to block the flood of water into the Imperial Valley that formed the Salton Sea (253' below sea level) in 1911.
A majority of the line was removed in the 1950s, especially south of the border in Mexico. The line from Niland to the border is still in use by Union Pacific.
It is said that train orders for the line were issued in both Spanish and English.
Thanks to Paul McGuffin for contributing information about this route.