The Yunnan–Haiphong railway is an 855 km (531 mi) railway built by France from 1904 to 1910, connecting Haiphong, Vietnam, with Kunming, Yunnan province, China. The section within China from Kunming to Hekou is known as the Kunming–Hekou railway, and is 466 km (290 mi) long. The section within Vietnam is 389 km (242 mi) long, and is known as the Hanoi–Lào Cai railway. The railway was built with 1,000 mm gauge due to the mountainous terrain along the route. Currently it is the only main line in China using 1,000 mm metre gauge.

The Red River, also known as the Hong River and Sông Cái in Vietnamese, and the Yuan River in Chinese, is a 1,149-kilometer (714 mi)-long river that flows from Yunnan in Southwest China through northern Vietnam to the Gulf of Tonkin. According to C. Michael Hogan, the associated Red River Fault was instrumental in forming the entire South China Sea at least as early as 37 million years before present. The name red and southern position in China are associated in traditional cardinal directions. The river is relatively shallow, and carries a lot of reddish silt along its way, appearing red brown in colour.