When I stayed overnight I Williams, AZ in Feb. 2010, we had 3 trains go by on the Phoenix line in 12 hours. 2 Z trains & 1 Autorack, with only one of the Z train WB. Not a great indication of the line's traffic, but a snapshot nonetheless.
For those who don't know, Williams is about an hour West of Flagstaff, and the BNSF transcon splits with the line to Phoenix east of town at a spot known as Williams Jct. To the west of there, the transcon follows the Crookton Cutoff, while the Phoenix Line follows the origional Transon alignment until almost Ash Fork before turning south. The Crookton Cutoff is a spectacular example of re-alignment of a railroad. The line has far less steep grades and gentle curves, a stark contrast to the old Williams-Ash Fork Line.