Communication by rail brought the most significant of the single changes into the Northern States. Before the panic of 1857 the trunk-line railways had completed their net of tracks between the Mississippi and tidewater. Nearly ten thousand miles had been built in the Old Northwest alone in the ten preceding years. But the effect of this on business, certain to come in any event, was not seen until secession closed the Mississippi to the agricultural exports of the Northwest. For a part of 1861 and 1862 traffic piled up along the young railroads extending from St. Louis and Chicago to Buffalo, Pittsburg, New York, and Philadelphia. But before 1863 these lines, notably the New York Central, the Erie, and the Pennsylvania, had adapted themselves to the trade which the South had thrust upon them; and never since secession has New Orleans regained her place as the great outlet of the Mississippi Valley. The fundamental change in the direction of its trade added to the prosperity of the North. In the additions to the transportation system, made to accommodate the new business, new railroads were less prominent than second tracks, bridges, tunnels, and terminal facilities. The experimental years of railroading had passed before most of the lines learned the importance of city terminals. The growth of the cities and the rising price of land made the attainment of these more difficult than they need have been, while city governments and their officials learned that illicit profits could be made out of the necessities of the railroads. The great lines, active in the development of their plants, and consolidating during the sixties to get the benefits of unified management, added to the bustle in the cities in the North. [Illustration: THE RAILWAYS OF THE "OLD NORTHWEST" Showing the development between 1848 and 1860, upon which the Civil War prosperity of the region was based] The United States was an agricultural country until the beginning of manufacturing and the revolution in communication made it profitable to concentrate people and capital in the cities. Between 1850 and 1880 the number of cities with a population of 50,000 more than doubled. The actual construction of the houses, the water and lighting systems, and the sewers for these communities gave employment to labor. As cities grew, their more generous distances brought in the street-car companies, whose occupation of the public streets added to the temptations and opportunities of the officials of government. The swelling manufactures increased the city groups and gave them work.