I wonder why they covered those old bridges. nice pics.
The reason was for maintenance purposes.
back in the day they didnt have the pressure treated lumber and the weather would quickly rot the wood decking on a bridge, Covering the bridge kept the water off of the decking and kept it from rotting out as quickly.
The cost to maintain the bridge was reduced enough to more than cover the additional cost of covering.
Its also why if you notice, the entrance to the bridges is always up grade, that was designed to keep the water flowing away from the opening to further protect the surface.