In the late 1800s, the growth of the railroad system led Midwestern cities such as Minneapolis and Cleveland to become industrial centers. By the end of the 19th century, a railroad network expanded significantly, linking cities such as Cleveland, Detroit, Minneapolis, and New Orleans. The impact of the railroads on Midwestern cities favored westward expansion; but, at the same time, it brought remote parts of the country into a national market economy where old industries expanded as well as new ones emerged. So, the correct option is "industrial centers".