Narrow Guage RR's went up the Platte Canyon and they also went up Clear Creek (ala Georgetown Loop). Much of the Clear Creek RR Grade is visible in Clear Creek through CC Canyon, and in the Platte it is the trail we walk on in Waterton. Goes through Bailey Canyon too. (Another went up S. Boulder Creek but I don't think it was ever finished and used but it is also along the river through Eldo and above to Gross Res)
In fact they went all over the place. The same RR (I believe) that used to go up through Waterton ended going up along the Ark on the East Side of the River between Buena Vista and Leadville. It was a bummer when Chafee County abandoned the road because a really nice set of camp and access opportunities were lost for paddler access, particularly between Rapid V and Pine Creek.
This has info about the RR up the Platte: Platte Canyon - Wikipedia, the free encyclopedia