Final answer:
The railroad was the major form of transportation that linked the east and west coasts of the United States, becoming the dominant mode of transport by the 1850s and playing a crucial role in the nation's economic expansion and market revolution.
Step-by-step explanation:
The major form of transportation that linked the east and west coast was the railroad. The construction and expansion of railroad infrastructure began in the 1830s and had a transformative impact on the American economy during the 1840s and 1850s. Railroads connected port cities with the interior and eventually became the predominant means of transport, surpassing canals, which were the main route of commerce prior to the advent of rail travel. The growth of railroads, coupled with the emergence of a national market system, provided stability to the growing nation and facilitated its governance over expansive territories. By the time of the Civil War, the United States had established an extensive system of railroads, which included more than three thousand miles of canals and thirty thousand miles of railroad track.
The Erie Canal, which opened in 1825, was a significant project that connected the Northwestern territories to global markets, but by the mid-19th century, railroad construction had gained even more momentum. The development of this transportation infrastructure, such as the completion of four transcontinental lines by 1887, was critical for the market revolution and the economic growth of the United States, enabling the cheaper and more efficient shipment of goods across the country.