I don't have much factual information to contribute to this conversation, but I'd like to offer a subjective impression. I visited Flagstaff briefly about 10 years ago and was blown away by the frequency of traffic on the former AT&SF there. There was literally a doublestack every 15 minutes, day and night -- truly amazing. My companions complained that they weren't able to get much sleep because of all the trains (we were staying at a hotel near the tracks), but I loved it. That area of northern Arizona should be on every railfan's list of places to visit. (Of course Flagstaff is an attractive city in it own right, and the Grand Canyon is nearby as well, so it has other things going for it in addition to trains.)
-Philip Marshall