There are no really curved decks or girders. Instead, such bridges comprise segments of straight decks with planking or decking of varying dimensions to allow for a generally curved function.
In the case pictured above, the deck is undoubtedly wider than usual, perhaps by as much as 6' to keep passing engine parts well away from pedestrians and the girder flanges. Of course, it depends on the degree of curvature. The view above is foreshortened due to the low angle. If the actual deviation from tangent at the centreline is only 1', they may have gotten away with a standard bridge size.