In the center of an old railroad bridge in Reynoldstown, a man pedaled a unicycle, arms outstretched. An odd-looking chap, he had spindly fingers made from motorcycle foot pegs and a red taillight heart that gleamed, E.T.-like, under horseshoe ribs. Visitors to last year’s Art on the Atlanta BeltLine exhibition could bring him creaking and clacking to life with a separate set of foot pedals. Will Eccleston’s Uniman is gone now, dismantled in the artist’s backyard, just as the overgrown grass and rusted tracks will someday be transformed. But for a moment, Uniman was part of an unfolding history.