From the Parks Department's website:
From the [Hell Gate Bridge] proper stems the viaduct – a long arcade of . . . high Romanesque arches made of sandy-colored concrete, recalling the form and grand march of a Roman aqueduct. The route beneath the arcade has been converted to the new Hell Gate Pathway, a dedicated pedestrian and bike path that runs through the monumental arcade, connecting to the larger waterfront pathway system that circles [Randall's and Wards Islands].
Fabulous pic.