From YT: Around 450 people participated this year, making it atleast 20 times bigger than last years. If youre unfamiliar with the Subway Dance Party, its a very easy concept. Once on the subway, a couple people will begin dancing while listening to music on their mp3 players. At first they get weird looks, and a lot of people laughing at them. Slowly though, more people begin joining in on the dance. Over the next couple stops the dance party gains in size, until the whole subway is in motion, and yet this is all done silently!

Another clip of this posted by krupo here:
http://www.videosift.com/video/February-Flashmob-Subway-Dance-Party-09-Toronto