Before you even start building a route like this, I highly recommend building something small even if it's fictional. Once you've got these skills under your belt, you can move on to bigger fish. I'm not trying to discourage you here, but giving you something to contemplate. If you feel you can do this kind of project so be it.
For something like this, you will want to obtain TransDEM.
TransDEM (rolandziegler.de) TransDem is not free, but costs about $26 give or take, based on what I recall from the last time I looked.
TransDEM will allow you to import the actual DEM data for this area of Colorado, place 1:24000 Topographic maps in place, and export this as a route ready for laying track. You can if you want have the program place track along the ROW where you mark it, but that's optional. Once you've got the route into TANE, you can then edit it and place your trees, buildings, track, and other assets to make a blank landscape into an actual route.
The thing with TransDEM is it'll allow you to create prototypical terrain easily once you've mastered the steep but doable learning curve. There are many, many options available including importing old GEOPDF files, placing of tiles, and other things that make route building relatively easy once the skills are mastered.
In addition to this part of the project, it's highly recommended you contact the D&RG historical society for any information that you may want, need, or can obtain. You will need plans for buildings unless you can find off the shelf ones such as those by Colorado71 whose buildings are simple but well done and well suited for this route.
