A rapid means of long-distance transportation became a necessity for the United States as settlement (新拓居地)spread over 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 I860 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.
　　The major problems with Americans railroad system in the mid 19th century lay in.
　　A. poor quality rails and unreliable stopping systems
　　B. lack of financial support for development
　　C. limited railroad lines
　　D. lack of a transcontinental railroad

來源:可可英語 http://www.ccdyzl.cn/cet4/201405/296278.shtml