This is the "uncensored" version of Improv Everywhere's amazing April Fool's prank video, The No Underwear Subway Ride. This video shows how the group pulled off the hoax. Although they claimed that 1,000 people had exposed their genitals on the train, they actually just filmed 20 actors wearing skin-colored underwear, and then pixelated the footage to make them look naked. The "uncensored" version of the video embedded below will show you what they actually looked like.
And, OMFG, but I recognize most of these stations -- I know where they were filming (like, close to where I lived)! Performance by Improv Everywhere. This is an annual event, the most recent was celebrated in 43 cities in 16 countries, wow! Here's more video, links and plenty of hilarious comments from previous events.
Those passengers' expression looks hilarious.