Cecil B. DeMille takes us back to the 1860s, then rebuilds the first intercontinental railroad in Union Pacific. The real-life spectacle is occasionally interrupted by the fictional adventures of railroad overseer Joel McCrea, postmistress Barbara Stanwyck (with an incredible Irish brogue), and McCrea's best pal Robert Preston. Unfortunately Preston has fallen in with Brian Donlevy, who is dedicated to destroying the Union Pacific railroad on behalf of a crooked political cartel. During an Indian attack, McCrea and Preston fight side by side to save Stanwyck, prompting Preston to turn honest. On the day in 1869 that the "Golden Spike" is to be driven at Promontory Point, Preston is killed saving McCrea from Donlevy's bullets. Union Pacific owes a great deal to John Ford's 1924 film on the same subject, The Iron Horse, even restaging one or two major action sequences from the earlier film. This DeMille spectacular was a big hit with audiences of 1939, who craved a booster shot of flag-waving now and again.
~ Hal Erickson, All Movie Guide