From: Hessen, Germany - now living in France
The railroad is missing but it appears to have been constructed after the Khalkan Gol / Normanhan battle.
The first railway line in Mongolia was a narrow gauge (750mm) line connecting the capital, Ulaanbaatar, with the coal mines of Nalaikh, some 33km distant. It opened in 1938. [Note: This railroad is missing as well]
During World War II, Japanese forces occupied Manchuria in northern China. The Soviet Union saw this as a threat and despite a non-agression pact signed between Russia and Japan in 1941, built up a strong military presence in eastern Mongolia, then effectively a satellite country of the Soviet Union. There were three bases at Sanbeis, Matad and Tamsagbulag. The bases were constructed in secret and much of their history was unknown until recently. Only in 2015 did a team of Japanese archaeologists confirm that the three bases were served by a railway. The Russian (1520mm) gauge railway diverged from the Manchurian branch of the Trans-Siberian Railway at Borzya and crossed into Mongolia to serve the bases. Its total length was around 400km, and it appears to have been completed in 1943. A section of the line as far as Choibasan was retained after the War, and appears to be still active today. From 1988 to 1993 there was a branch north from Choibasan serving a uranium mine at Mardai.
The printed source "Area Handbook for Mongolia" indicates that the Borzya-Choibasan line was Russian broad gauge and that narrow gauge (1 meter) spur lines were build 1940-1941 around Choibasan "both for strategic and economic reasons".