If you can drive through Winslow about 50 miles east of Flagstaff by taking the business loop off and back onto I-40, it's worth a few minutes to walk through the trackside La Posada, a restored Harvey House hotel and a Ssouthwest Chief stop. There's a very nice restaurant in the hotel, but train viewing inside the restaurant is limited to looking out windows and eating there would use likely eat up a fair amount of time (and money). Winslow is a crew-change point for BNSF so the trains are going slow going by the hotel as they either drift into the yard or power up.
Closer to Flagstaff, if you exit I-40 at Cosinio Road, go a very short distance north and turn off onto the dirt road right before the crossing so you can look down on the double tracks. You can get decent shots of eastbounds with the San Francisco Peaks (the mountains that are just to the north of Flagstaff) in the background as they approach and enter the cut. I try to get up there annually.