I am very intrigued to see someone attempting anything Brightoian, as my great, great grandfather was a signal man at Brighton for most of his life.
I found 6mm ply is too thin for a baseboard top unless you aim to cross brace every 4 inches or so.
Base boards can be made to fit together by tacking on one extra end frame to one end and setting the other back, kind of male and female. It's easy to do, makes for simple assembly of layouts at exhibitions and does away with the need for alignment fixings.
Regards Shaun.
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