A fascinating view of a 'replacement' NS bridge at Portageville NY under construction. Of particular interest to me are the temporary horizontal cables keeping the new construction from falling into the gulch, and the "movable bed" that the mobile cranes on each working face of the bridge are resting on.
The drone photo is posted here: [www.railroad.net
Scroll down a bit and click the photo to see full size detail.
Presumably the minimal decking is an effort to keep the weight as low as possible until the two halves are connected. My guess is that the supporting cables will be used to jockey each half a little bit to facilitate the final connection when the two halves finally meet.
Contractor's page on this bridge: [www.americanbridge.net