The first transcontinental telegraph (completed 10/24/1861 ) was a line that connected the existing network in the eastern US to a small network in California, by means of a link between Omaha (NE) and Carson City (NV)., via Salt Lake City. It was a milestone in electrical engineering and in the formation of the US. It was primarily the work of the newly-founded Western Union Telegraph Company (1851; merged with another in 1856).
Studio publicity noted that Fox contract star Henry Fonda had served as technical adviser on the film, due to his experience as a young man working as a lineman. Fonda's advisory capacity was most probably a publicity fiction and in any event, Fonda was not credited on the film itself.
Originally, Laird Cregar was cast in an undetermined role (possibly that of Doc Murdoch), but was unable to do the film due to an unfinished project. He was replaced by George 'Gabby' Hayes, but Hayes then became ill and was himself replaced.
The $5,000 Western Union paid to buy back its own stolen horses in 1860 would be worth over $141,000 in 2019 after factoring in the effect of inflation.
After he arrives in Omaha, Richard Blake (Robert Young) has a stranger watch his horse and buggy. Blake comes out again and the man chastises him, telling him he is the Provisional Governor of the Territory of Nebraska. Based on this being set in 1860 when the transcontinental telegraph was being established, the Governor of the Nebraska Territory was Samuel W. Black. Col. Black was killed 6/27/1862 during the Civil War.