The Rio Grande had a sprawling network of main and branch lines throughout Colorado and Utah. Any attempt to understand these lines must start to break the network down into some sort of groups. Any railroad with well over 100 years of history is going to undergo expansions, contractions, and significant reorganization. This makes using the railroad's own designations impractical, since routes were continuously being moved between divisions or whole divisions consolidated. So, instead I'm going to organize this history by main route segments.
These segments will be primarily chosen based on the fullest extent of the Rio Grande system. Understandably then, these segments will be more biased towards the later, standard gauge network. For the meantime, I've broken things down into two basic categories:
D&RGW Overall System Map - Not all smaller lines shown
Also, here's a list of locations on the D&RG(W) lines that have individual pages.