Above are two pictures of the Circus Maximus. The picture on the left is a model of what the Circus probably looked like in the 1st and 2nd centuries CE. The picture on the right is a modern picture of the Circus. The long, open plain is one side of the track and the thin strip of grass between two lines of dirt is where the center divider probably stood.
Left photo from: http://www.ancientsites.com/er/circuses.html