Inside the structure, edge-to-edge is only 2 1/4 inches. It's only 1 3/8 between the walkways on the sides, where the track goes. As I recall, the walkways are applied separately and could be omitted. But, the top girders are a single piece, and to widen the bridge to highway size would require an entire new top section.
It could be a one lane road bridge with signs at each end to warn motorist.
A traffic signal at each end to control occupancy would be an interesting sometimes used prototypical feature.