Railroad allows the building of Darwin's Voyage and railroads, which increase troop movement between cities substantially and increases that square's production.
It is a prerequisite of industrialization.
Civilopedia entry[]
The Railroad resulted from the successful marriage of a promising new technology, the Steam Engine, and a growing need: fast, safe, and inexpensive transportation. Railroads applied the unprecedented power of the locomotive to the pulling of trains of loaded wagons over rails. This resulted in a dramatic increase in the amount of cargo that a few men could move quickly and over great distances. Railroads were a critical contributor to Industrialization. They not only made it easier to obtain raw materials and reach markets, but the search for better equipment spurred innovation that benefited many other industries.
Trivia[]
Curiously, in Sid Meier's Civilization, railroad is written with the second R capitalized.