"A system of transportation using special vehicles whose wheels turn on metal bars fixed to the ground, or a particular company using such a system." (CD)
See also: https://dictionary.cambridge.org/us/dictionary/english/railroad
The graph displays the other subjects mentioned on the same pages as the subject "Railroads and Railroad Companies". If the same subject occurs on a page with "Railroads and Railroad Companies" more than once, it appears closer to "Railroads and Railroad Companies" on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.