The Trans-Mongolian line follows an ancient tea-caravan route from China to Russia. It connects Ulan-Ude in Russia to Jining in China, cutting across Mongolia from North to South via Ulan Bator. The Trans-Siberian also stops in Ulan-Ude, which allows a potential connection. There are other important stops, like Sukhbaatar, Darkhan, Choir, Sainshand, Zamiin Uud and Erlian in China. There are also important branches leading to Erdenet and Baganuur.
The line was built between 1949 and 1961. Mos part of Mongolia has single track, whereas China has double track. The gauge is 1520 millimetres (4,11 feet) in Russia and Mongolia, and 1435 millimetres (4,8 feet) in China, so it must be changed at the Chinese border, which takes about 2 hours.
Railway developed quite late in Mongolia. Before inaugurating the railway in 1961, there were only two small railways. In 1938, the first railway, 50 kilometres (31 miles) long, connected Nalaikh and its coalmines to Ulan Bator. The second railway was built by the Soviets and was inaugurated in 1949 ; it was an extension of the Russian railway connecting Ulan-Ude to Naouchki that was built nine years earlier. Because diplomatic relationship were getting better between Soviet Union and China, it became possible, in 1950, to start the construction of the line connecting Ulan Bator to China. It was inaugurated in 1961 but very soon was closed because of a diplomatic crisis between USSR and China. It could reopen only 20 years later, in the 1980’s.