BNSF Seligman Sub through Flagstaff,AZ

I will be going on vacation August 20th-26th to the Grand Canyon and taking rails to the rim as well, I would like to know how often BNSF trains pass through Flagstaff on weekdays and weekends because my Dad most likely won't want to wait there forever we would probably only be able to stop for a few minutes and I would also like to know some good spots in the town of Flagstaff,I dont want a schedule I just ant to know about how long we would need to wait for something to show up and what are my chances of seeing a BNSF SD70ACe.