Questions 36 to 40 are based on the following passage:
A rapid means of long-distance transportation became a necessity for the United States as settlement spread ever farther westward. The early trains were impractical curiosities, and for a long time the railroad companies met with troublesome mechanical problems. The most serious ones were the construction of rails able to bear the load, and the development of a safe, effective stopping system. Once these were solved, the railroad was established as the best means of land transportation. By 1860 there were thousands of miles of railroads crossing the eastern mountain ranges and reaching westward to the Mississippi. There were also regional southern and western lines.
注：1.首句提出问题，后跟着解决方案。
四级文章套路：有现象必有解释，有结论必有解释，有问题必有解决方案
serious 对应36题中的 major problem，最高级出现是考点
The high point in railroad building came with the construction of the first transcontinental system. In 1862 Congress authorized two western railroad companies to build lines from Nebraska westward and from California eastward to a meeting point, so as to complete a transcontinental crossing linking the Atlantic seaboard with the Pacific. The Government helped the railroads generously with money and land. Actual work on this project began four years later. The Central Pacific Company, starting from California, used Chinese labor, while the Union Pacific employed crews of Irish laborers. The two groups worked at remarkable speed, each trying to cover a greater distance than the other. In 1869 they met at a place called Promontory in what is now the state of Utah. Many visitors came there for the great occasion. There were joyous celebrations all over the country, with parades and the ringing of church bells to honor the great achievement.