Thanks for all the responses and ideas folks. I do have the book and get ideas from the Diablo Canyon Bridge. As Bill says, this is N scale and it will be built out of a combination of corrugated cardboard and cereal board. You are right Errol in that only the abutments will be fixed in place before the scenery is done. The bridge itself will then be installed.
The main beams are made of corrugated cardboard with a combination of with grain and cross grain. Here is the first step of cut pieces.
Clark, the first thing is to have a very sharp cutting knife. I use one that has the break off sections of the blade. Make the first cut lightley, and take about 4 or 5 cuts a little harder each time then the corrugations don't crush or twist
Thanks very much for you kind words folks.
One of the benefits of N scale is that details are so small that they are almost invisible so it is quite OK to leave them out and as Shaygets says, one doesn't miss them.
Here is the structure complete ready for painting.