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.
Corrected entry: One of the Graboids dies from running into a large saw, partially embedded in the ground. The problem is judging by the size of the Graboid in relation to the flimsey, thin saw, there is no way the creature would have died from the impact. And the saw shows little damage.
Correction: We don't know the exact biology of the Graboids. As far as we know, the point of impact could well have been it's brain or heart, or other major organ that would be vital to sustain it's life.
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