Well now, GNGoat, ya got me ta thinkin'... The line to Roy is the northern remnant of the old NP Prairie Line, which was the original mainline between Tacoma & Tenino, WA. The Prairie Line originated in downtown Tacoma & ran up the gulch along South Tacoma Way & the " Nalley Valley ". This line continued past the South Tacoma Shops to Lakeview , where the American Lake line branches off & runs along Interstate 5 to a connection with the Point Defiance line @ Nisqually.The Prairie Line has been severed on the North end by the Sound Transit " Link " light rail line that runs up Pacific Ave. in downtown Tacoma. Tacoma Rail now serves both the line to Roy, as the Lakeview Industrial Spur, and the former American Lake line, as the Lakeview Line. The only connection to BNSF is @ Nisqually. I do not know if current operations involve interchanging w/BNSF @ Nisqually, or Tacoma Rail trackage rights from Tacoma.
BTW, the Roy line runs through the artillery range on the east side of Ft. Lewis, and an old BN timetable tells of the need to contact the base range officer for permission to pass through the area...