Well, there never was a PRR bridge over the Muskingum river in Marietta. The only railroad bridge over the river is one that was always owned by the B&O. The PRR track always ended on the east side of the river.
I was born and raised in Marietta, so I am quite familiar with the railroads that were/are there. You should be able to easily find some photos of the B&O bridge. It has been converted to a walking trail. Now and then, they still open the bridge to move the W.P. Snyder steamboat in and out of it's docking site for repairs. In fact, the boat is due to return to Marietta any day now, so the bridge will have to be turned. I bet you can find video of the bridge on youtube.
Jeff