Hey there Mezzoprezzo, I see you are a fellow 2004 Trainzer. Good to know that there are still some of us around.
Thai10n, my advice to you is contact Ben Dorsey for he is the bridge king and master creator of all kind of bridges. He just recently made me a two track wooden trestle that looks and works just great. Between the two of us, we worked out the track spacing so that Auran's double track will attach to it. I think that a steel & concrete version of a two track bridge is well within his reach. He will actually make what ever you want, make sure it IS what you want and not stop until you are 100% satisfied. He does request pictures and/or plans for requests but with what you have I'm sure he can work with you, especially if you can get more pictures of that bridge above.
Cheerz