Hello JohnH,
You will enjoy riding the remnants of the narrow gauge lines in Colorado. I have been luck enough to ride both the Durango & Silverton Narrow Gauge Railroad and the Cumbres & Toltec Scenic Railroad. In fact my brother and I spent a week as volunteers working on the C&T back in 2001. The C&T has a wonderful volunteer program and I hope to do it again in the future. Although the D&SNGRR is the more famous of the two railroads, make sure you experience the C&T. It has a more historic feel to it in my opinion.
Should you find yourself in my neck of the woods, I recommend you visit the Cass Scenic Railroad in West Virginia. It's a few hours drive north of my home in North Carolina. Cass isn't narrow gauge but if watching Shays battling up a 10% grade is your thing, it's not to be missed!
http://www.durangotrain.com/
http://www.cumbrestoltec.com/
http://www.cassrailroad.com/