A friend showed me a picture of stone bridge abutments or similar remains on Pine River next to Pine River Pond Road in Sanbornville, NH. Was there ever a rail line there parallel to the road, or are we just looking at an old road bridge?
It seems very unlikely that these are railroad related. There are/were two railroads in the Sanbornville area. One was the B&M line from Rochester to North Conway. While some of it has been abandoned, the part from Rochester to Ossipee Pit is still in use by the New Hampshire North Coast for gravel trains. The other railroad was the Wolfeboro branch of the B&M. The remains of that also retain a rail use; it is used for "speeders" operated by the Cotton Valley rail trail club.
Neither of these current day uses would leave "stone bridge abutments or similar remains," and neither parallels the Pine River Pond Road as far as I can see from the Google maps.
-John