Durant mentions that he owns 20 acres on either side of the rail line. However knowing, with his shady deals, he was worth several million dollars at the time, 20 acres is insignificant. Land grants were offered at 20 miles on either side of the railroad route, so the "20" figure Durant talks about would need to be explained further.
When the train is attempting to cross the frozen river, a number of shots show the front of the engine towards the rear of the train, and the track beyond. These shots appear to follow the river up- or downstream, not from bank to bank. So it appears that the train and track follow the river, rather than crossing the river at its banks. The bluffs and presence of trees also appears to indicate this. When the train breaks through the ice, the track is perpendicular to the flow of water, which is correct.
John Campbell was appointed territorial governor by President Grant, who did not take office until March 1869, yet the season 4 of the show has him arriving in 1868, before Grant had even been elected.
Season 4 of the show has John Campbell arriving in 1868 and all events caused by him happening that year, before Grant had even been elected.John Campbell and the other federal appointees who arrive with him were appointed by President Grant in March 1869, shortly after he took office.