The "Grand Paris Express" railway network is currently under construction. It has been very quickly predicted that piston effect created by train operated at 110 km/h (68 mph) through a succession of 1,000-2,000 m tunnels and stations with a cadence of 90 seconds in rush hours will differ from the traditional Parisian metropolitan network. The piston effect created in front and behind the train has been estimated to a total pressure variation of +900 Pa / -1400 Pa in a few seconds. The piston effect will be reflected in the stations and in the shafts positioned roughly every 800 m in the tunnels for the intervention of emergency services. These shafts are also used for ventilation, first to control smokes and protect the stations in case of fire in the tunnel, then to ensure air quality in the tunnel when natural ventilation is not sufficient. As a consequence, the equipment in the shafts will face with piston effect and will be subjected to strong variations in pressure. At reduced speed, for comfort use, it is expected the fans will be passed through with a negative airflow and operated in negative pressure. Therefore they will be operated in areas that are not defined by the usual. In order to characterize a "Grand Paris Express" type fan, Eiffage carried out a test bench to test the train piston effect. This test bench is made up of a circuit in which is inserted a Ø1000 mm model fan that has the same aeraulical and mechanical characteristics of the 300 fans that will be installed in the service structures of this new network. The model fan has been first tested in the unexplored area of negative pressure and negative flowrate to define the behavior in the three quadrants of the curve. It is now operated with successive negative and positive forced pressure on hundreds of thousands of cycles to ensure the durability of the equipment on several years of operation.