Well, in Colorado, there are lots of abandoned railroads.
Here is a large number of the major ones.
1. Argentine Central, ran to from Silver Plume to Waldorf and to the top of Mt. McClellan, was later renamed the Argentine and Grey's Peak. Abandoned in 1918, tracks removed in 1920. Narrow Gauge
2. Book Cliff Railroad (I think this is the Mule Tram) Abandoned in th 1910's or 20's (I think). NG
3. Colorado Midland RR, Abandoned in the 1920's. Standard Gauge
4. Rock Island, Abandoned in the 1980's. SG
5. Colorado Central (Later became part of the C&S) Can ride over portion of this line on the Georgetown Loop. Abandoned in the 1930's. NG
6. Midland Terminal, Abandoned 1949. SG
7. Gilpin Tramway, Abandoned in 1917 (There is a Gilpin Shay for Trainz). NG
8. Colorado & Southern (Narrow Gauge Portions), Abandoned from the 1920's to the 1940's. NG
9. Colorado Springs & Cripple Creek District Railway, Abandoned in 1920. NG
10. Laramie, North Park & Western (Line later bought by the Wyoming Colorado Railroad), Abandoned in 1987. SG
11. Denver & Inter-Mountain Railroad, Abandoned 1953. SG
12. Denver, Leadville & Gunnison (Bought the DSP&PRR in 1889), Abandoned in the 1930's to 40's. NG
13. Denver & Salt Lake (Formerly Known as the Denver, Northwestern & Pacific Railway), Rollins Pass Line Abandoned after the opening of the Moffat Tunnel in 1928. SG
14. Denver & Rio Grande Western, (Narrow Gauge and some Standard). Large majority of the narrow gauge abandoned in the 1950's. Standard: Tennessee Pass line closed in 1997, Glenwood Springs to Aspen, Abandoned in the 1980's. NG & SG
15. Denver, South Park and Pacific, Abandoned in the 1930's. NG
16. Crystal River Railroad, Abandoned in the 1940's. SG
17. Florence & Cripple Creek, Abandoned in the 1920's. NG
18. Georgetown, Breckenridge and Leadville (Later to be one of the Georgetown Loops many formal names), Abandoned in 1939. NG
19. Silverton Railroad (Rainbow Road), Abandoned in 1926. NG
20. Rio Grande Southern, Abandoned in 1951. NG
21. Silverton, Gladstone & Northerly, Abandoned between 1938 and 1942. NG
22. Silverton Northern Railroad, Abandoned in 1942. NG
23. Southern San Luis Valley Railroad, Abandoned in 1957. Some track remains and is being rebuilt by the San Luis and Rio Grande (Rio Grande Scenic). SG
24. Uintah Railway, Abandoned in 1939. (Mallet 51 is also in Trainz). NG
25. Various Trolley and Electric Traction Lines. (Such as the South Denver Cable Railway and Denver Tramway) (The former Trolley House of the former is now used as an Old Spaghetti Factory and the latter's powerhouse is now an REI). SG
26. Various UP Lines. SG
27. Various BNSF Lines. SG
There is still lots of evidence of the right of ways of many of these railroads found throughout colorado, whether it be bridges, fills or ROWs. Some are open for tours and some have become dirt roads open to the public.