What city's metro system is called "the world's longest art gallery"?

Below Stockholm is the world's longest art exhibition...or so the city's metro has been called. Started in the '50s, the 70 miles of metro is one colorful art history course. Stockholm made a conscious decision to create art in the public sphere, as they believed art should be more accessible, rather than only for the bourgeois. And it seems to have worked!

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