Railroads can be built only by settlers, and only in tiles that already contain a road, after the RailRoad technology has been researched. To create a railroad, move a settler unit to a tile with a road and press the R key. Units moving from one railroad tile to another railroad tile use no movement points, regardless of terrain. Railroads increase food and resource production in a single tile by 1/2 (rounded down). They increase trade by the same amount, but before the effects of Republic or Democracy governments.
Civilopedia entry[]
Railroads revolutionized transportation by providing a relatively cheap and fast method of moving people, raw materials, finished goods, and military units. Railroads were essential to the Industrial Revolution. Not only did they support and encourage industrial growth by dramatically reducing transport costs, but they spurred technological research on their own. Advances in locomotive technology and engineering spun off into other industries and applications. Railroads were the first great industrial corporations.