USA HISTORY

Detailed explanation-1: -Each company faced unprecedented construction problems-mountains, severe weather, and the hostility of Native Americans. On May 10, 1869, in a ceremony at Promontory, Utah, the last rails were laid and the last spike driven.

Detailed explanation-2: -The Central Pacific met its greatest challenge at the outset-the towering Sierra Nevada, which presented enormous engineering obstacles and strangling winter snows. Deep fills, rock cuts, high trestles, snaking grades, and 15 tunnels through 6, 213 feet of solid granite blooded the CP crews.

Detailed explanation-3: -Answer and Explanation: For the US government and the railroad companies, the biggest obstacles in building the Transcontinental Railroad were mountains of solid granite and attacks by Native American war parties.