Cullen is in California with the Central Pacific Railroad, and must learn to contend with the new workers; word comes in from Salt Lake City about the Hatches.
Cullen enlists new workers to help him move a locomotive on a sled; Cullen and Fong must work together; Swede and Phineas fight for survival after being banished to the high country.
Cullen must deal with the violence in Chinatown, caused by disgruntled workers; Phineas is manipulated in his plot against the Mormons.
In the wake of the violence, Cullen solves a Chinese labor strike; Mickey is pulled deeper into Union Pacific Business.
Violence erupts during the festival of Tin Hau Don in Chinatown; Cullen confronts the Swede; Durant gets Maggie involved 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.
When the Swede goes on a murderous rampage on the Hatch homestead, a long-overdue showdown with Cullen ensues.
Cullen considers life-changing decisions in Utah; Truckee adapts to life without Cullen; lies compromise Durant's new partner.
Cullen's nitroglycerin experiment makes an explosive backdrop for the railroad life in Truckee; Durant's plan takes a deadly turn.
Mickey has blood on his hands after working with Durant; Campbell returns to town, complicating Louise's life.
Chang forces Mei to leave Truckee after discovering her secret; Cullen exorcises some old demons during his search.
When the race to finish the railroad comes down to mere inches, all parties find themselves pondering the end of the line and what the future has in store for them.
Government hearings, incalculable actions and the golden spike breed new beginnings and endings for those left standing.