Photo taken by Anthony Dillon in April 2010
BH Photo #160546
Unfortunately Stone Masonry is mostly a long-lost art.
Corrugated metal and/or concrete supplemental arches are often used to preserve arch rings.
In some cases, entire arches are encased in concrete for longevity's sake.
Looks as if there is a metal culvert beneath the arch. I wonder if that was laid in after it was built and grouted in as a reinforcement in a repair effort?
Here's a video I recently did on the bridge: https://www.youtube.com/watch?v=9C1BtY6i3_I