Completed and joined the two deck plate bridge sections and am ready to start working on the center support tower. Building the second bridge was clearly easier than the first and finished in a little better shape. That being said, the novice probably can't tell which one was finished first.
I thought about how to join the two halves and settled on using three angled plates of .080" ABS which is very stiff. I made a cardboard template first and actually tried out the angle on the railroad itself before cutting plastic. But even then, the angle was two sharp and I had to adjust the first piece so it matched the angle on my plan.
These plates were glued to the flanges on the inner I-beams that hold the bridge together. I glued them on one side and then when it was "sort-of" set, brought the other piece into contact. I then applied more Bondene liberally to ensure a good bond. The end result was a very strong junction that just needed some trim pieces to hide the insides.
When fit over the tracks it fit exactly as planned. I used some more styrene, and ABS sheet and angles to fill the gap.
For the center mounting plate, I shaped and then glued on a piece of .080" ABS upon which the bridge shoes will be fixed. For the outside ends, rather than stacking up some more ABS, "H" beams glued crossways serves as a strong support.
The center places also increased the strength of the joint significantly.
The entire assembly is solid and feels like one piece. When set on the end beams, it's very stiff and self-supporting.
Now it's onto the tower that will support the middle.