The train might move in a different direction each time you look at it. Stare long enough and you can make the train change directions. There is no correct answer, because this .gif is only four frames long; the optical illusion is in your brain. However, there is that one guy who recognized the subway station and just knew which direction the train comes there. -via Geeks Are Sexy
The gif above does show that motion blur has no 'head' and 'tail,' as many imagine it does.
If the train was travelling away from us, the light would be green or not exist.