It is easy to access. Technically you will be walking the train tracks which are private property. We hiked down from Arthesula falls and came down the Frankenstein trail and came across this trestle. But you can walk from the parking area along the tracks straight to the bridge.

And like the comment below. This is probably a drone shot.

Edit. Go to Wiley House. Walk right on the tracks (when approaching from the parking lot).

Edit2: this photo is not Frankenstein trestle. And the directions above will take you to Frankenstein Trestle