BNSF's Needles Subdivision, is a double-tracked steel raceway that runs west from Barstow, California, to the small town of Needles, near the Arizona border, and to my knowledge, it's the busiest mainline in North America, as it averages 80 to 90 trains a day.  This freight-only line sees lots of container, double-stack, piggyback and manifest trains, and the this pike is truly on the fast track, as the top speed limit, for certain trains, is 70 m.p.h. in some parts!  Quite a railroad!

Here are the scanner frequencies you'll need to railfan BNSF's busy Needles Subdivision:

160.935  Needles dispatcher, used from East Barstow to Needles
160.590  Needles dispatcher, used from East Barstow to Needles
160.245  Cadiz PBX, used from Danby to Amboy
160.260 Goffs Butte PBX, used from Homer to Goffs and Newberry to Barstow via repeater link
160.335  Hualpai Mountain PBX, from Needles to Homer, and via repeater at Ludlow, from Cadiz to Newberry

Note that Union Pacific trains have trackage over BNSF's track between Barstow and Daggett, and use BNSF radio frequenies when they're on BNSF's track.

Sure, there are yard frequencies for Barstow and Needles yards, but I rarely, if ever, listen to yard traffic, so those frequencies aren't included in this guide.  I generally stay away from yards, as I prefer scenic, wide-open spaces.