I mentioned that I'm building a viaduct, and one of the problems I've had has been getting hold of good quality detail drawings and the like, so in case it is of any use to anyone I've uploaded some scans of a facsimile drawing from an engineering journal of 1864 showing details of Digswell viaduct in Welwyn.
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The bit that's most useful, to my mind, is the sections through the pillars and trackbeds.
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Guy