The Onawa trestle is easy to find, though getting there is a somewhat longish (albeit very pretty) trip through the williwhacks. Easiest route: talke Routes 6/15 to Monson. A short distance past Monson village, Elliotsville Road veers off to the right, descending a very long, gradual hill. Take it. Continue on for about six miles or so (mind you, that's a guess). Eventually you will cross a concrete bridge over a stream. Immediately past the bridge, you have a choice of turning right or continuing more or less straight, up a hill. To get to Bodfish, continue straight. You will quickly come to the MM&A's Bodfish crossing. There is a section house there (still used) with a sizeable parking lot. If, after exploring a bit, you want to continue on up the road, you'll eventually come to a pleasant spot called Drew Valley, just to the west of Borestone Mountain. You can turn around there.
To get to Onawa, take the aforementioned right at the concrete bridge. (You're still on Elliotsville Road.) Continue a short distance and take your first left, which is a dirt road. This is the road to Onawa. Eventually you'll cross the RR track and reach the village. Park your car in the designated parking area, and feel free to explore to your heart's content. Until the early '70s Onawa was a bustling little place. There are still full-time and part-time residents there; the store and depot are gone, but the former school and Catholic chapel are still standing, the latter used as a sort of community center.
The Onawa Trestle is down the track (easterly) about a half-mile from the village. I need to warn you that the track is posted. You are, thus, taking a risk. I don't know to what extent, if any, the MM&A prosecutes people for walking out to the trestle. I do know that a lot of people do it --- it's impressive, it's one of New England's most famous railroad bridges, the site where it's located is magnificent, and for generations it has been something that Mainers and out-of-staters have enjoyed seeing. There is a walkway along the north side of the bridge, with a couple of safety / lookout platforms.
