In this sentence, Berger refers to how transport and industrial activity expanded heavily over this time period when many Americans were moving out west. This is a massive part of North American history because of the opportunities it opened up for rich and poor people alike. The first railroads in North America were established in the 1820's and quickly altered the face of life on the continent. The rapid expansion of the railroad industry led to the development of other industries like oil and steel. For a time in the 19th century these were the largest economic factors in the United States and whoever controlled these essentially controlled the country. However, Berger here is referring to the opportunities that railroads opened up for expansion across the west and north. The incredible development of railroads made travel to the west coast of America much easier than it ever was before. This changed the idea of the "frontier" in many people's mind. Because travel was much easier, the idea of the west in North America being a rustic and dangerous environment changed overtime and culminated in western economic opportunities like the Gold Rush. "Gold discoveries in the Far West preceded the gold
rush of 1849 and encouraged migration from eastern cities." As western movement became more and more popular, the idea of a pristine, wild, and natural American west began to fade. The idea of a getting rich quick from the California Gold Rush was translated to the Klondike Gold Rush in the Yukon of Canada in 1896. This had similar affects on how people viewed the Northwest region of Canada and its wilderness.
"Railroad Promotion and Economic Expansion at Council Bluffs, Iowa, 1857-1869." Annals Of Iowa 42, no. 5 (Summer1974 1974): 371-389. America: History and Life with Full Text, EBSCOhost (accessed April 6, 2017).