Question

What is the main mountain range in eastern Mexico that begins south of the Rio Grande?

Answer 1

The main mountain range in eastern Mexico is the Sierra Madre Oriental

Explanation:

The Sierra Madre Oriental starts at the Rio Grande and runs for more than 600 miles south to the east-west transversal mountains of central Mexico.

The Mexican Gulf Coastal Plain lies to the east of the range, between the mountains and the coast of the Gulf of Mexico.

The Mexican Plateau lies to the west of the Sierra Madre Oriental mountain range, between the mountains on the east and the range of mountains in western Mexico.

According to Wikipedia,
"The Sierra Madre Oriental is part of the American Cordillera, a chain of mountain ranges (cordillera) that consists of an almost continuous sequence of mountain ranges that form the western "backbone" of North America, Central America, South America and Antarctica."
https://en.wikipedia.org/wiki/Sierra_Madre_Oriental

Here is a map of the Sierra Madre Oriental mountain range

http://meetingmexicotareaintegradora.blogspot.com/2016/02/knowing-mexico_26.html