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Polyjim, A truss bridge usually has an 'aspect ratio' of between 1:5 to 1:10 and yours should be closer to 1:10. The aspect ratio is the ratio of the depth to the span. So your deck truss bridge will need to be at least 2.5 inches deep. Do you have or could you aquire the book Model Railroad Bridges and Trestles published by Kalmbach? If you have this book I can look up several pages to describe the features you would want to build into your bridge. I recently bought a Central Valley single track truss bridge which spans about eighteen inches. If you are modeling in a very early era (after 1880 but before 1900) then you could probably buy two of those kits and cut them in the right spot to kitbash a 24+ inch span. What era are you modeling in? The reason this is important is because the later your period, the heavier the design load and the more substantial the structure would have to look to be realistic. Are you wanting this to be a duck under or a lift out section? I know you said Duck Under but I want you to recognize all your options. You may not want to go this way, but here is one option I would consider because it would be impressive for your guests. I would build a 'false bridge' by placing a piece of wood over the gap. Then I would build the 150 ft Central Valley thru truss bridge I mentioned and two Micro Engineering 50 ft deck girder bridges. I would build the bridges and arrange them as 50 ft - 150 ft - 50 ft. This arrangement gives you plenty of width for your duck under passageway. Then you would glue these to the wood to conceal it. Then you can add two piers and two abutments by gluing them from below. Finally, you can add sceanery below the bridge. The total height would not have to be more than about three inches below the track and this is about the depth of the deck truss anyway. There are two reasons I would recommend this. First, the sceanery and the bridge together make a bigger impact than just a bridge crossing an empty space. Second, (and this is just my preference) I think the mundane subject makes a better model than the spectacular. This may sound odd to you. I think that when you model the mundane, your audiance recognizes the item and subconsciously fills in details that you may not have actually built into the model. Also, if you have gone the extra mile to detail a subject, these efforts are recognized by the audience as well. Now there is one situation where you would not want to build a multi-span bridge as I described and that is if you are modeling in the rugged mountains. In that case, only a single span bridge would look right. I can help you build that one too if that is the case. Regardless, there is one thing I recommend you get right now and that is a piece of Micro Engineering 36 inch bridge flex track (Code 83 rail). It is needed to help make your bridge look right and many visitors will notice the track is different from the regular mainline track and you can explain that bridge track requires longer ties spaced closer together. If you like, you can buy two pieces of code 70 rail and make the guard rails as they would on an important prototype bridge. I imagine this bridge will be in a good location for your visitors to see, and I am happy to help you make it a real feature for your visitors to remember. - Ed
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