wig-wag
n.— «The banjo crossing signal, also called a wig-wag, was (and still might be in some places) a round target with a hole in the center that swung back and forth when activated. Looked somewhat like a banjo being swung back and forth by its neck.» —“Re: What’s a Banjo Eye?” Usenet: rec.railroad May 10, 1990. (source: Double-Tongued Dictionary)
Further reading
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