I keep meaning to post about this - the Vat Run. Construction's been under way for a while, should be opening next year. There's a working party most Sundays, and they do trail-building courses there too.
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Also, the walkway on the west side of the Forth Road Bridge is open every Saturday to let people have a look at the works for the new bridge. Makes a change from the east side - and it may be my imagination, but I think it's bouncier!