It is the planned extention of the RER E.
The central undeground part would link Nanterre la Folie, La Defense, Porte Maillot or Perriere at Haussmann Saint Lazare and Magenta (Gare du Nord, link in 2012 at Gare de L'Est). The underground part between Magenta and Haussmann Saint Lazare was inaugured in 1999.
Outside the underground part the RER E would link Mantes la Jolie in the west at Meaux and Tourman in the east.
Actually the RER E run between Haussman Saint Lazare in the center and Chelles Gournan/Tourman in the east.
Picture by Pline
Picture by Omar Omar
Picture by Eole99