The Heber Valley RR (Std guage) is located 45 minutes east from SLC. They are currently running diesel off season, two active steamers are in the shop (ex UP 2-8-0 #618, and ex GW 2-8-0 #75). Golden Spike is also off season, the two 4-4-0's are having the jewelry polished this time of year. Ogden Museum is a good visit.
For Rabid D&RGW fans, there are still a few SD40T-2's in Grande paint working the hill out of Helper, and Utah Railway just took delivery of 6 MK5000's fresh out of the paint shop.
Frankly, you don't have enough time to do all there is to do!