Factual error: Though extremely modest on today's standards, the dress worn by Clara to the hoedown shows far too much cleavage for the time. No schoolteacher would ever wear a dress like that in the 1880s.
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)
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: 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: 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.
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: Sierra #3, the locomotive used in BTTF 3, is a 1891 Rogers, despite this movie being set in 1885. (00:40:45)
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: In shots of the town outside of the saloon in 1885 a modern day California flag can be seen. This flag was not used until 1911.
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.