Beginning in the early 1870s, railroad construction in the United States increased dramatically. Prior to 1871, approximately 45,000 miles of track had been laid. Between 1871 and 1900, another 170,000 miles were added to the nation's growing railroad system. Much of the growth can be attributed to the building of the transcontinental railroads. In 1862, Congress passed the Pacific Railway Act, which authorized the construction of a transcontinental railroad.