A great time to visit Maine this year will be for the National Narrow Gauge Convention, in Portland Maine. You'll be right down the street from Maine Narrow Gauge Museum, and only an hour away from the WW&F and Boothbay Railway museums. The Sandy River & Rangely Lakes museum is about 3 hours away.
We at the WW&F are planning on running trains all during the convention (hopefully steam-powered trains), and there'll probably be something special going on at Boothbay Railway and MNGRR.
The convention is, as always, the end of August/first of September. Usually the hot weather has passed by then, but with El Nino or Global Warming or whatever else is out there, who knows!