Craig,
The original D&RG (minus the "W") connected Denver with Grande Junction and west to the Colorado border--in narrow-gauge--via Pueblo, Salida, Marshall Pass, and Gunnison A subsidiary called the Rio Grande Western, extended the narrow-gauge tracks to Salt Lake City via Soldier's Summit. Later the Rio Grande Western was absorbed into the Grande, and that's how the "W" became part of the corporate name.
Almost immediately afterwards, the Colorado Midland started building directly west out of Colorado Springs in standard gauge; this put the fear of God into the Grande, and they started laying down the third rail as fast as they could. Somewhere in all of those goings on, the Grande figured they would be a whole lot better off if they went north from Salida with the third rail to Leadville (instead of over Marshall Pass). Then, using what was then just a spur to the zinc mines at Gilman, they pushed the third rail over Tennessee Pass. When they got to Gilman, they pressed on with standard gauge only, basically cutting the Midland off at Glenwood Springs and winning the standard gauge battle for Grand Junction. It is my opinion that when the Grande finally reached Salt Lake City with the third rail, it was the start of the demise of the Grande's narrow-gauge system. Dozens, if not hundreds, of books have been written about the Grande and tell the story better than I can. What I have relayed is just a capsule summary.
The importance of the Tennessee Pass line was further diminished when the Grande acquired the Denver & Salt Lake RR and eventually built the Dotsero Cutoff between the Craig line and the old main line, lopping about 250 miles off the Denver-Salt Lake trip. Tennessee Pass became a bridge route for traffic going east at Pueblo to Kansas City (via the MpoPac), but it was no long the heart of the Rio Grande, and as for the narrow-gauge, well, it just became the albatross hanging around the neck of what was now a standard-gauge railroad.
Whew!
Mike
