The huge expansion of rail lines in midwestern United States during the 1850s had major economic and environmental effects.
By 1850 the United States possessed roughly 9,000 miles of railroad track; ten years later it had over 30,000 miles, more than the rest of the world combined. Much of the new construction during the 1850s occurred west of the Appalachian Mountains—over 2,000 miles in the states of Ohio and Illinois alone.
The effect of the new railroad lines rippled outward through the economy. Farmers along the tracks began to specialize in crops that they could market in distant locations. With their profits they purchased manufactured goods that earlier they might have made at home. Before the railroad reached Tennessee, the state produced about 25,000 bushels (or 640 tons) of wheat, which sold for less than 50 cents a bushel. Once the railroad came, farmers in the same counties grew 400,000 bushels (over 10,000 tons) and sold their crop at a dollar a bushel.
The new railroad networks shifted the direction of western trade. In 1840 most northwestern grain was shipped south down the Mississippi River to the bustling port of New Orleans. But low water made steamboat travel hazardous in summer, and ice shut down traffic in winter. Products such as lard, tallow, and cheese quickly spoiled if stored in New Orleans' hot and humid warehouses. Increasingly, traffic from the Midwest flowed west to east, over the new rail lines. Chicago became the region's hub, linking the farms of the upper Midwest to New York and other eastern cities by more than 2,000 miles of track in 1855. Thus while the value of goods shipped by river to New Orleans continued to increase, the South's overall share of western trade dropped dramatically.
A sharp rise in demand for grain abroad also encouraged farmers in the Northeast and Midwest to become more commercially oriented. Wheat, which in 1845 commanded $1.08 a bushel in New York City, fetched $2.6 in 1855; in similar fashion the price of corn nearly doubled. Farmers responded by specializing in cash crops, borrowing to purchase more land, and investing in equipment to increase productivity.
As railroad lines fanned out from Chicago, farmers began to acquire open prairie land in Illinois and then Iowa, putting the fertile, deep black soil into production. Commercial agriculture transformed this remarkable treeless environment. To settlers accustomed to eastern woodlands, the thousands of square miles of tall grass were an awesome sight. Indian grass, Canada wild rye, and native big bluestem all grew higher than a person. Because eastern plows could not penetrate the densely tangled roots of prairie grass, the earliest settlers erected farms along the boundary separating the forest from the prairie. In 1837, however, John Deere patented a sharp-cutting steel plow that sliced through the sod without soil sticking to the blade. Cyrus McCormick refined a mechanical reaper that harvested fourteen times more wheat with the same amount of labor. By the 1850s McCormick was selling 1,000 reapers a year and could not keep up with demand, while Deere turned out 10,000 plows annually.
The new commercial farming fundamentally altered the midwestern landscape and the environment. Native Americans had grown corn in the region for years, but never in such large fields as did later settlers who became farmers, whose surpluses were shipped east. Prairie farmers also introduced new crops that were not part of the earlier ecological system, notably wheat, along with fruits and vegetables.
Native grasses were replaced by a small number of plants cultivated as commodities. Corn had the best yields, but it was primarily used to feed livestock. Because bread played a key role in the American and European diet, wheat became the major cash crop. Tame grasses replaced native grasses in pastures for making hay.
Western farmers altered the landscape by reducing the annual fires that had kept the prairie free from trees. In the absence of these fires, trees reappeared on land not in cultivation and, if undisturbed, eventually formed woodlots. The earlier unbroken landscape gave way to independent farms, each fenced off in a precise checkerboard pattern. It was an artificial ecosystem of animals, woodlots, and crops, whose large, uniform layout made western farms more efficient than the more-irregular farms in the East.
A选项中的 stopped by the Appalachian Mountains 原文没有提到，不对。
C选项说的因果关系不符合原文，原文没有具体说 the value of goods arriving in New Orleans increased 的原因；
F选项对应原文第六段， Native Americans 选项不对，主要是原因没说对，不是因为 the varieties they planted had far poorer yields，而是因为 The new commercial farming fundamentally altered the Midwestern landscape and the environment。