The story is set post-civil war, but all the major characters carry Colt Peacemakers with 4 3/4 inch barrels. This model was developed for the US Army in 1873. Civilian sales started in 1875, and the 4-3/4 barrel length wasn't available until 1877.
The film shows the railroad being completed into Santa Fe, NM. The AT&SF railroad never went to Santa Fe (despite the name). Although the main line did not reach Santa Fe, a branch line was constructed to stop there.