Sounds like you have your answer, depending on exactly how high your duckunder is (you didn't specify.) I just got done with a project that literally flipped my layout 180 degrees, to turn a duckunder into a lift-out bridge, and it's already making my model railroading life much easier (and my back feels much better!) I still have to build some guard rails on the sides, but the actual building of the bridge was far easier than I thought it would be. I spent years overthinking and planning for elaborate latches or elevating sections before using an idea from a Lance Mindheim blog post: carriage bolts under and alongside the bridge make for an adjustable foundation.
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