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.

Mengzi is a city in the southeast of Yunnan Province, China. Administratively, it is a county-level city and the prefectural capital of the Honghe Hani and Yi Autonomous Prefecture, located about 243 kilometres (151 mi) southeast from Kunming, and 400 kilometres northwest from Hanoi, Vietnam. It is situated in the centre of a fertile valley basin on the Yunnan-Guizhou Plateau 1,310 metres (4,300 ft) above the sea level and was home to about 590,300 inhabitants as of 2021 census. Mengzi was formerly Mengzi County (蒙自县) until October 2010, when it was upgraded to a county-level city. Mengzi is the core area of South Yunnan Central City Cluster, which is officially regarded as the political, economic, cultural, and military centre of South Yunnan.