As Jim notes, you have a good start with your mock-up. I'd use styrene to build the finished model, .060" sheet for the sides and the deck, and .030" for the underside of the arches. One thing that I would change would be to extend the sides up about 2' higher than the deck, making sure that your longest cars will still clear it, particularily on the inside of the curve. That will allow you to ballast the deck, which was the usual practice with this type of bridge. To add some character, laminate two 2' high strips of .060" to the inside (track side) of each extension, and a 3' or 4' high strip to the outside top edge, then make a cap for both of these side walls out of more .060" sheet.
To construct the arches, after you have cut the outside faces of the bridge, you'll need to provide some solid support for attaching the .030" material in place. Scraps of the .060" will work fine for that: simply cement them to the backside of the bridge sides, but set-back .030" from the edge. That way, the upper (unseen) edges of the .030" material will be glued to the edges of the .060" scraps, and the edges of the .030" styrene will be cemented to the back faces of the bridge sides. When the glue has dried completely, use a file or some sandpaper to smooth the joints.
You
could sand the entire bridge with a medium sandpaper, then paint it a suitable concrete colour and be done. Or, if you want a stone finish, you could scribe the styrene, before assembling the components, to represent cut stone. To widen the grooves, use a pair of pliers to snap the tip off an old X-Acto #11 blade, then drag the backside of the blade through the scribed lines.
There are also many commercial "stone" sheets available. If you can find them in styrene, use the same cement to laminate them to the bridge as you used to construct the bridge. (I use lacquer thinner as styrene cement: cheap, and fast drying.) If the stone material is a different kind of plastic, such as ABS or vinyl, contact cement will work well, and is also useful for heavy embossed paper "stone" sheets.
By the way, always nice to see another TH&B modeller. :thumb:
Wayne