Well, its not exactly in the Boston Metro, but the abandoned overpass in the Georgetown -Rowley State forest over I95 would probably qualify.
It theoretically carries Pingree Farm Road, but you wouldn't want to try it - it's really a hiking trail. There's not that much online about it, but it appears built to vehicular standards given it has guardrailed approaches, storm drains and a sidewalk, but has likely never carried road traffic. Here's a video, skip to 2:20 or so to see the bridge itself.
It theoretically carries Pingree Farm Road, but you wouldn't want to try it - it's really a hiking trail. There's not that much online about it, but it appears built to vehicular standards given it has guardrailed approaches, storm drains and a sidewalk, but has likely never carried road traffic. Here's a video, skip to 2:20 or so to see the bridge itself.