I was surprised and delighted to find the Monson No. 3 under steam in Phillips, Me., last summer. I had seen it operating a number of times at the Maine Narrow Gauge in Portland, but I never really believed I'd see steam in Phillips. The SR&RL has an impressive roundhouse there, on the site of the original building known in later years as the "old stone fort," plus a short but pleasant run through the woods.
If you're going to be in Maine, it's worth visiting not only the Maine Narrow Gauge, in Portland, and the Sandy River, in Phillips, but also the Wiscasset, Waterville & Farmington, outside of Wiscasset, and the Boothbay Railway Village, in Boothbay (which has German 0-4-0t's, along with some of the nicest staff members you'll ever talk to -- although, come to think of it, just about everyone I've ever talked with at a two-foot line in Maine has been friendly and has taken time to answer every question I've been able to come up with). The WW&F and Boothbay can be done in the same day if you're careful with your scheduling; Phillips, though, is a good hike to the west.
The pic is of Monson No. 3 approaching the Sandy River's station at Sanders, trailed by a coach and a caboose.