It's a public artwork by artist and photographer Bill Brand which combines a 300-foot long piece of art with a special enclosure that has 228 narrow slits that are surrounded by backlighting. As the train moves by at about 30mph, your eyes only see the art through the slits, making it appear as if it was animated.
The Masstransiscope was originally installed in 1980, restored in 2008, and again in 2013 after damage from Hurricane Sandy.
To see the installation, simply look out the right side of your train car when heading towards Manhattan on a train leaving DeKalb Avenue.
via Untapped Cities
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