Factual error: When the Indians are running from the cavalry they cross over a set of tire tracks. They are too far from where Marty started so cannot be his, and the lines can't be wagon tracks because the lines are too wide and the width is uniform. (00:19:30)
Factual error: In 1885 when Maggie McFly asks Seamus for a word, they walk into the other room and behind them are a few garments hung on pegs on wire hangers. The wire hanger was invented by Albert J. Parkhouse in 1903, eighteen years later. (00:25:50)
Factual error: Sierra #3, the locomotive used in BTTF 3, is a 1891 Rogers, despite this movie being set in 1885. (00:40:45)
Factual error: Preparing to run the model train in Doc's workshop, Marty connects the clamp to the positive terminal. But he actually connects to the white ceramic insulator. Doc correctly connects to the negative metal terminal. (01:10:01)
Factual error: Each time one of the special Presto-logs begins to burn the locomotive suddenly accelerates. In real life the logs would heat the water faster and generate more steam, but there would be an appreciable time-lag between the time the logs begin to burn and the engine accelerates. Also, train engines must accelerate slowly. If too much power is applied the drive wheels merely spin on the rails. (01:35:20)
Factual error: When the red Presto-log is burning, rivets begin to blow out of the locomotive, supposedly from the pressure. The rivets in the boiler, which would be subjected to pressure, are not visible on the outside of a locomotive. The rivets which are seen to blow out in the movie are in the smokebox, an area which is subjected to heat and smoke, but no steam pressure. (01:40:10)
Factual error: At the end of the train scene just before Marty travels to 1985, the train busts through a road block warning that there's only 1/4 mile of track left before the ravine. Keep in mind that the train is travelling at approximately 70-80 mph by this time. At 70 mph (and not even accelerating) it would only take the train 13 seconds to reach the ravine. The actual time in the movie is at least a minute. (01:40:31)
Suggested correction: This can't really be classed as a factual error because we don't know how much of the track has been built since that barrier was put up. They could've put that barrier up when there was only 440 yards or so of track left, but have laid another 800 yards of track since then. I myself actually work on the roads; if we're resurfacing a road, we don't move the sign 100 yards further up the road every time we've resurfaced 100 yards. That sign stays there till we're done, regardless of the distance.
Factual error: Though extremely modest by today's standards, the dress worn by Clara at the hoedown shows far too much cleavage for the time. No schoolteacher would ever wear a dress like that in the 1880s.
Factual error: When Marty goes to meet Mad Dog Tannen for the duel, in the background on the right side is the current flag of California. But that flag did not become the state flag of California until 1911, so would not have been around in 1885.
Factual error: Standard gauge for railroad tracks in 1885 was 4ft 8 1/2 inches. The distance between the center of the wheels on a Delorean was 5ft 3 inches. The car would not fit on to the tracks, and we can see the wheels aren't recessed or otherwise altered.
Factual error: In 1885, Hill Valley and its surrounding areas are shown as having a desert climate, when in both 1955 and 1985, the climate is grassland. In reality, Nevada County, California (where Hill Valley is supposed to be located) is and always has been grassland and forests, not arid desert.
Suggested correction: Hill Valley is in Hill County, as stated at the activation of the clock in 1885.
Factual error: In the scene where Doc and Clara are playing with a telescope at night, the moon is seen twice. The phase of the moon appears to be a waning gibbous. Though the phase of the moon on the night of September 5th, 1885 at pacific time was waning, it was actually a waning crescent, a much different looking phase of the moon. I looked this up on the United States Naval Observatory website.
Factual error: The steam locomotive in the film is of a 4-6-0 wheel arrangement and is lettered as Central Pacific no. 131. The Central Pacific did have 4-6-0 engines in 1885, but according to the company's locomotive roster, the number 131 belonged to an engine of a 4-4-0 wheel arrangement. Also when the locomotive is uncoupled it shows a Janney (fist style coupler) - that style wasn't invented until 1890. The locomotive would have used a pin and link style connection.
Factual error: When Marty travels back to 1885 and meets the Native Americans, we see a view from inside the car with the digital speedometer. The car is still going over 70 when the Indians are less than 2 metres away from the front of the car. There is no way the car would be able to stop that quickly from that speed.
Factual error: When Marty arrives in 1885, he's heading towards the Indians at over 70mph with only a matter of meters between them. In the next shot, Marty has stopped the car and is reversing. Even on asphalt, Marty would need the length of two Olympic swimming pools to stop the DeLorean from 70mph. It would be impossible for him to come to a complete stop given the distance he had.
Factual error: After Doc and Marty hijack the train, there are shots of them in the cab of the locomotive. Look closely behind them, there is a clip to hold a microphone for a 2-way radio or CB attached to the window post, which shouldn't be there in 1885.
