Corrected entry: Jet engine vapour trails are visible in the sky on 2 separate occasions, once when the team discover Old Freds abandoned stage coach, and again near the end of the movie with the final shootout with the last Graboid. The movie is set in 1889.
DavidRTurner
29th Oct 2011
Correction: These are not necessarily jet vapours (contrails). The one at 0:46:50 is possibly a cloud - it looks fairly straight-line, as a contrail might look as it dissipates, but clouds can look like that. We cannot assume this to be a contrail. In the other scene, I assume the poster means the one behind Hiram at 1:30:35, as I cannot spot another. This one is just a telegraph wire.
DavidRTurner