From the 1925 Clarksburg and Weston quads:
Monongahela Valley Electric ran parallel to the B&O tracks until they split at the bridge with the electric crossing the bridge and the B&O going along the east side of the creek.
They still look pretty beefy for an interurban and the fact that they don't "match" suggests reuse. The mining in the area a century ago led to a lot of trackage that's no longer present. It's likely that as lines to played out mines were taken up their bridges were moved.
Looks to me like one span of a traditional road bridge was paired with a repurposed railroad span.
It's definitely narrow enough to be an older railroad bridge.
I am pretty sure this is originally a railroad bridge. Not only does it look like a railroad bridge, I can see remnants of the RR line on aerial imagry where the highway breaks away from the old RR alignment.