Contrary to what is depicted throughout this series, Wells Fargo did not operate a stage coach line. They used state coach lines for express shipments of money, gold and other valuables but did not operate the lines themselves.
Since the pilot episode, outdoor shots including the sky have frequent incidences of jet contrails on otherwise cloud free days. Sometimes appearing singly, sometimes by handfuls. And sometimes, the sky is clearly very busy;presumably due influence by Phoenix Sky Harbor International Airport or Luke Air Force Base. All of which didn't exist in the days of The Old West.