Factual error: The movie takes place in 1868, and every guy and his dog are wielding cartridge conversion Army, Navy, and even what appears to be Walker colts. Unfortunately, such conversions were not performed until 1869, after Smith and Wesson's rights to the Rollin White patent expired.
Factual error: While the film is supposedly set in the late 19th Century, the locomotive is oil-fired. The tender carries neither wood nor coal. There is no way the Mexican National Railroad would possess oil-burning locomotives in this time period.