America’s first steam locomotive made its debut in 1830, and over the next two decades railroad tracks linked many cities on the East Coast. By 1850, some 9,000 miles of track had been laid east of the Missouri River. During that same period, the first settlers began to move westward across the United States; this trend increased dramatically after the discovery of gold in California in 1849. The overland journey–across mountains, plains, rivers and deserts–was risky and difficult, and many westward migrants instead chose to travel by sea, taking the six-month route around Cape Horn at the tip of South America, or risking yellow fever and other diseases by crossing the Isthmus of Panama and traveling via ship to San