"At the approach of the mid-nineteenth century, some immigrants were drawn by the availability of land in the vast reaches of North America. Economic development also offered opportunities beyond agriculture for newcomers. Industrialization created jobs in mills and as manual laborers in cities. The expansion of railroads was another major force attracting immigrant labor. In 1830, the United States had a total of only 23 miles of railroad tracks. Only one decade later, this figure had grown to 2,818 miles. It rose to 9,021 miles in 1850 and 30,626 miles in 1860. Immigrants from Ireland played a particularly significant role in laying new railroad tracks. Economic hardships and political disorders in the sending countries also helped stimulate emigration to the United States. The most significant event was the Great Irish Famine of 1845-1851, which was caused by a devastating potato blight. Irish were already the most numerous immigrants, and the famine drove even more of them to leave their homeland in the hope of finding relief in North America. In the various German states—which would not be united under a single government until 1871—a wave of failed revolutions in 1848 created a flood of political refugees who swelled the ranks of German Americans."