I'd echo that a trip up to Animas Forks would be my choice if only one day was available. You will return with a true understanding of the tenacity that Otto Mears demonstrated in getting the SN up the mountain!
I found 4 days totally inadequate to explore the the RR right of ways and some of the mining roads in the area even on my trail motorcycle.
Most of the roads are well defined, but you need to get a bit emboldened for some of the creek crossings.
If you can beg, borrow or steal a copy, THE RAINBOW ROUTE is a mandatory read before your journey. Robert Sloan and Carl Skowronski present 416 pages of well researched history and photos of the three Railroads. ( The book is a bit pricey...saw a dealer asking $90 for a train show this spring, but in my opinion it is a must have!)
You might prowl around the following links, if the book is out of reach:
[
www.narrowgauge.org] Mark Evans does a good job producing and maintaining this one
Also, although not entirely RR related the Red Mountain Project warrants following [
OCS.fortlewis.edu]
Gosh: Looks like I have just talked myself into going back to Silverton!