Maybe Trains Magazine has a West of the Mississippi edition full of Eeastern subjects and an East of the Mississippi edition full of western subjects.

Although I live in the East, my favorite railroads are mostly in the Western states. Maybe I'm too easily satisfied, but I've read and enjoyed many Trains Magazine articles on Western railroad subjects. In addition to those listed by George Andrews, I've enjoyed the following:

The Joint Line, Moffat Route, SP's Cascades Line, the LaVeta Pass Line and the Sunset Route
Ben Bachmann's articles about Seattle railroading
844
The Northern Pacific as the "All American Railroad"
A 1997 article on railroading in the San Francisco Bay Area
A tribute to the SP (The Octopus is Sushi)
Cumbres & Toltec
Port of Tillamook Bay
An awesome article with detailed maps of Portland, OR area railroading (nothing like it was done for their hometown of Milwaukee)
Union Pacific's 150th Anniversary
BNSF's Raton Pass Line
UP's Tennessee Pass Line (wish it came out a year or two sooner to inspire me to travel to Colorado to railfan the line)
Los Angeles Union Station