This is a “telltale”. Today it’s over a gorgeous rail trail, but once it was over a train track of the Northern Railroad in New Hampshire.
I don’t think I would have noticed it if I had not learned about it from the New Hampshire Rail Trail book by Charles F Martin.
He says: “These are horizontal bars holding a set of vertical metal strips that hang down toward the track. If a train worker were on one of the cars and couldn’t see the bridge coming, then a telltale would tickle his back, reminding him to keep down”