Rail transport in the Soviet Union the Soviet Union collapsed. Twenty other stations were under construction in 1985. The country's rapid transit system was the most intensively used in the world. The Soviet Union was a pioneer in the development of the diesel locomotive to replace the steam locomotive (dieselization) on non-electrified lines. But the USSR failed to make steady progress and while they lead the United States at first, they soon fell behind and their last steam locomotives were retired about 15 years later than for the US. The first mainline diesel locomotive in the world began running in 1924 in the USSR but it
Treaty on the Creation of the USSR adjoined to the Soviet Union on 3, 5 and 6 August respectively. The final republic was the Moldovian SSR that merged the large territory of Bessarabia with the Moldovian ASSR previously part of the Ukrainian SSR. After World War II, no new republics were established, instead, the Karello-Finnish SSR was downgraded into an autonomous republic and re-annexed by the RSFSR on July 16, 1956. On December 8, 1991, the leaders of the Ukrainian and Belorussian SSRs, and the RSFSR met to agree on the annulment of the 1922 treaty, which was terminated on December 25, 1991, effectively dissolving the USSR.