The primary difference between the structures mentioned and the example in Maine is that they are bridges and not an overpass. Bridges generally span over a gap from one point to another connecting a separated grade i.e. the points connected generally are at a higher elevation than the elevation of the bottom of the gap.
The overpass in Falmouth, ME, is different because it connects two points which are at the same grade as the railroad itself. In essence the location could just as easily be served by a private crossing at grade but for the convenience of the railroad this was eliminated by building the overpass.
So in this sense the Falmouth, ME structure is very very unusual today as most of these types of overpasses have long since been replaced by at grade crossings or by grade separated overpasses.
Apparently it's really rare to see one of these relatively maintenance intensive structures still standing.
gokeefe
