the expansion work, with consisted of 2.3 kilometers of track and two new stations, began in february 1977 and took place mostly in open air. the two stations contain central platforms due to the narrowness of the rail network, spanning only twelve meters. this constraint required a specific type of construction so to assure the structural stability of the adjacent buildings with respect to the settling of the earth. the stable layer of chalk under the ground allowed for vaulted support structures. excavation began with a concrete-filled trench, and the tunnel containing both rails was connected at the buckle of "auteuil" by two tunnels each with their own track. for the first time on the parisian network, the terminus did not contain a post-arrival turnaround due to the lack of space, so the maneuver took place before the station. the opening of this section, which is entirely underground, took place in two phases: the first expansion to "boulogne-jean jaurès" was inaugurated on 3 october 1980, and the second section to "boulogne-pont de saint-cloud" was opened on 2 october 1981. boulougne at first was serviced by only every other train, with the second train returning eastward via the loop at "auteuil". after 6:40 pm, all trains serviced the entire line.