There are already some serious Civil War era modelers and lots of information to help you model in the appropriate 1861-65 framework. As others have pointed out, however, the "Old West" is largely a twentieth century fabrication, with many differing perspectives on what that meant. For the most part, as others have noted, railroads in the trans-Mississippi US were built after the Civil War in the 1870-1880s by which time railroading and its equipment were much changed from Civil War stock. Take a look at the following online sites for a sense of the difference as reflected in some superb modeling.
If you are serious about modeling in either of these these frameworks, you should also become a regular member of the Yahoo Early Railroador Civil War Railroad forums. Both offer a lot of information on historic prototypes, modeling techniques, and available kits and materials for these .
Be aware, however, that there are few accurate, ready to run models other than expensive brass (see especially SMR's line of excellent O gauge models for a high standard) for either of these periods. There is a good supply of freight stock kits in O and HO from suppliers like BTS, Alkem, Rio Grande models, Silver Crash, Amesville Shops, etc. But other than Labelle, there are few manufactures producing much authentic passenger stock. The situation is even more limited when it comes to locomotives. Though Bachmann still produces a late 1860s 4-4-0, it is crudely made and a poor runner. Out of production Rivarossi V&T engines are a bit better, but were built to a strange scale, closer to OO than HO and have deep flanges. Oddly, the situation may soon be better for N scale. Micro-Trains is planning to produce Civil War freight cars, Republic produces N scale clones of the BTS larger scale early freight stock, and Atlas makes both a 2-6-0 and 4-4-0 appropriate for the 1870s. There are also a growing number of era appropriate N scale cars and parts available from 3D printing outfits.
In any case, good luck in deciding--and keep us posted of your ultimate decision.