Cullen is in California with the Central Pacific Railroad, and must deal with the new workers.

Cullen and Fong find themselves in peril as they transport a locomotive over the Sierra Nevada mountains. The Swede gains Phineas' trust.

Cullen contends with violence in Chinatown set off by disgruntled railroad workers. Meanwhile, the Swede manipulates Phineas in his plot against the Mormons.

Unexpected violence erupts in Chinatown during the festival of Tin Hau Don; Cullen confronts the Swede; and Durant involves Maggie in his latest scheme.

Cullen and Mei escort precious cargo through the California wilderness; Eva nurses Louise back to health.

Cullen and Durant meet with President Grant to discuss the future of the railroad; the Swede puts his plan against the Mormons in action.

The Swede's murderous rampage on the Hatch homestead results in a long overdue showdown with Cullen.

Cullen faces life-changing decisions in Utah as Truckee adapts to life without him.

Cullen's nitroglycerin experiment provides an explosive backdrop for the railroad life in Truckee. Durant's plan turns deadly.

Mickey's dirty business with Durant leaves him with blood on his hands. Meanwhile, Campbell's return to town complicates Louise's life.

Chang discovers Mei's secret and forces her to flee Truckee. Meanwhile, Cullen's search for her brings new realizations while exorcising old demons.

The race to finish the railroad comes down to inches, forcing all parties to contemplate the end of the line, and what the future holds for them.

The golden spike, government hearings and unpredictable actions mean new beginnings and endings for the survivors.