The guy and I went out to North Point Park today. It's a really cool area.
There's a footbridge that goes over a railway coming out of North Station, the green line is visible in the distance, and there's duck boats all over. Once you cross the Miller River (on foot), you are under the Zakim Bridge, which is that locally-famous cable bridge, and then there's a series of locks for boats that you can walk over. Cool in general, especially cool for kids that love transportation.
At one point, the guy asked me to take a picture of the railyard from the North Bank Bridge, and I dropped the phone. It slid down the mesh, and surprise, there was a gap at the bottom that the phone went into. It probable fell through down to the train tracks, but there's a chance it got stuck somewhere under the footbridge.
We went down to see if we could see it, but when I got with a certain range of the tracks, a guy on a loudspeaker yelled at me to not go closer and to instead use the "million dollar bridge." I couldn't see him, but I yelled in his direction, trying to explain my situation. He either didn't hear or care.
Later, we figured out, he was probably in this little white watchtower, but again, the only way to get to that was to cross all the train tracks.
I let it go. Later at North Station, I asked an MBTA person how I could get it. He had no idea who ran that part of the tracks; they only knew how to get to tracks in North Station.
We had a good time for the rest of the day, visiting the locks and the playground on that side of the Miller, and watching even more trains on the bridge.
I could call the MBTA on Tuesday and ask about this, but I'm skeptical that anyone will know how to get at that weird section of track.