The most significant obstacle in building the transcontinental railroad was

Crossing the Sierra Nevada and the Rocky Mountains

Explanation:

The Transcontinental Railroad was a huge project that lasted from 1863 to 1869 and connected the US Rail network with the Pacific Coast Railroads. There were many obstacles but the most significant one was Sierra Nevada since it was very expensive to make the railroad pass through the mountains to reach the Great Basin. Later they discovered a way across the Sierras that would be suitable for the railroad.

your answer is

relations between the soviet union and the united states were driven by a complex interplay of ideological, political, and economic factors, which led to shifts between cautious cooperation and often bitter superpower rivalry over the years. the distinct differences in the political systems of the two countries often prevented them from reaching a mutual understanding on key policy issues and even, as in the case of the cuban missile crisis, brought them to the brink of war.

the united states government was initially hostile to the soviet leaders for taking russia out of world war i and was opposed to a state ideologically based on communism. although the united states embarked on a famine relief program in the soviet union in the early 1920s and american businessmen established commercial ties there during the period of the new economic policy (1921–29), the two countries did not establish diplomatic relations until 1933. by that time, the totalitarian nature of joseph stalin's regime presented an insurmountable obstacle to friendly relations with the west. although world war ii brought the two countries into alliance, based on the common aim of defeating nazi germany, the soviet union's aggressive, antidemocratic policy toward eastern europe had created tensions even before the war ended.

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