I've been hearing about this "totally cool stuff that you see in the T tunnel between Harvard and Central squares" for about a month now, and last night I actually took a subway ride and saw it for myself. Very innovative, although not new (and rather expensive). Here's Forbes' take:
"Sub Media, cofounded by Spodek and Matthew Gross in 1999, produces 15- to 20-second animated ads that appear on subway tunnel walls. This is accomplished through a series of backlit pictures which Spodek compares to "the frames in a film reel." The pictures, printed by the U.K.'s Photobition Group on Kodak transparencies, spring to life as the train speeds by, creating a fluid film and giving passengers the effect of being inside a giant flip book." Check videos on their site to get the idea.