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For anyone who hasn’t been to the Gowanus Canal lately, the scene may come as a shock: Tall apartment buildings line the banks of the formerly fetid industrial waterway, with plywood construction sheds and the clang of foundation-digging heralding more on their way.

After the work crews clock out, other sounds take over: classic hip-hop from Bluetooth speakers, kids on bikes pretending they don’t hear their adults, and, where the sidewalk ends, an acoustic playground for old-time music players.

Bluegrass beckons on Wednesday nights in summer as circles of friendly banjos, mandolins, guitars and fiddles hand off showy solos, hel

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