I use balsa wood slats that I buy at Michael's. I either use a section of Luan for the face of the deck bridge, or in the case of the following pics, the subroadbed spline itself.
I found an old pic of the subroadbed spline that acted as the basis and provided the faces of the deck bridge.
This pic shows a piece of balsa that I cut to length for the uprights. I use a pair of locking forceps to hold it. I apply a thin bead of Super Glue on it's edge. It attaches to the face instantly.
I use a piece of a paint stirring stick as my spacer gauge as I glue successive uprights.
I leave a small space from the top of the slat to the top of the deck facing for a long piece of balsa that I glue with the flat side against the tops of the uprights. The bottom slat is done the same way. A little patience is needed here to let the Super Glue bond while I bend the slat around as needed.
That done, I then glue another slat on top of the first slat. This second slat is glued on with the narrow edge on top of the wide edge of the first slat. It creates a rail to contain the gravel. I don't glue down ballast as a rule because it's heavy and it won't really go anywhere. Also, if I want to make a change to the track by - say, putting a track trip in later, that is easy to do if the ballast isn't glued down. Gluing down ballast when using vintage steel track is problematic. The water used in the gluing process rusts the track.
When it's all glued up, I go check my emails or something for a few minutes to let the glue finish curing. Then I come back and spray wet-on-wet spray paints. I use cheap flat black and Rustoleum primer. I like the color combination.
And here is the finished facsimile of a deck bridge, all dry, ballasted and the track rails cleaned off.
Total build time - given that the face was already in place - about one hour. You may notice a few imperfections on the deck facing. That is no accident. I distress the facing a little bit before I begin the process, if it isn't already so. Since I don't bother with super-detailing rivets and such, I like the defects and feel that they add a "weathered" or "old" look to the bridge. There may be some who will mention that a curved deck bridge is utterly preposterous in prototype. The weight and vibration of the train would tear it apart. And they would be right. Deck bridges that span a curve are really made up of several or more sections of straight decks upon which curved track is laid, with pillar supports underneath. This is a toy hirail layout. I doubt my family will ever notice. Shhhhhhhhhh! Don't let on. lololololololololol