Chris,
The difinitive book is "The Switzerland Trail of America" by Crossen. The second edition is pretty common and around. I know Boulder Public as well as Denver Public Library have copies of it. If you can't get to there I'd suggest asking your local librarian about an inter-library loan.
Another great source is the western history collection of online photos: Search for particular names of the railroad companies that owned the line:
Greeley, Salt Lake & Pacific
Colorado & Northwestern
Denver, Boulder & Western