Passage: The ancestors of Yakuts were Kurykans who migrated from Yenisey river to Lake Baikal and were subject to a certain Mongolian admixture prior to migration in the 7th century. The Yakuts originally lived around Olkhon and the region of Lake Baikal. Beginning in the 13th century they migrated to the basins of the Middle Lena, the Aldan and Vilyuy rivers under the pressure of the rising Mongols. The northern Yakuts were largely hunters, fishermen and reindeer herders, while the southern Yakuts raised cattle and horses.In the 1620s the Tsardom of Muscovy began to move into their territory and annexed or settled down on it, imposed a fur tax and managed to suppress several Yakut rebellions between 1634 and 1642. The tsarist brutality in collection of the pelt tax (yasak) sparked a rebellion and aggression among the Yakuts and also Tungusic-speaking tribes along the River Lena in 1642. The voivode Peter Golovin, leader of the tsarist forces, responded with a reign of terror: native settlements were torched and hundreds of people were killed. The Yakut population alone is estimated to have fallen by 70 percent between 1642 and 1682 because of the Muscovite expeditions.In the 18th century the Russians reduced the pressure, gave Yakut chiefs some privileges, granted freedom for all habitats, gave them all their lands, sent Eastern Orthodox missions, and educated the Yakut people regarding agriculture. The discovery of gold and, later, the building of the Trans-Siberian Railway, brought ever-increasing numbers of Russians into the region. By the 1820s almost all the Yakuts claimed to have converted to the Russian Orthodox church, but they actually retained (and still retain) a number of shamanist practices. Yakut literature began to rise in the late 19th century, and a national revival occurred in the early 20th century.

Question: The creation of what transport system led to large numbers of Russians coming into the region?
the Trans-Siberian Railway