Well, I'd like to think that my guide to the PRB contains lots of
useful background information, too, including a timeline of the
development of the PRB.
http://www.robl.w1.com/prb-guide/index.htm
Though produced in 2000, the basic information still applies, though some of
the lines that were double track are now triple track, and some of the lines
that were triple track are now four tracks.
Instead of trying to revise the whole book, I've provided five pages of updated
information.
-- Ernest
P.S.: My book was designed for use in conjunction with the Altamont Press
Mountain Plains railfan TT, which, unfortunately has been unavailable
in the past few years.