The aerodynamic merits and demerits of cross-ventilating adjacent tunnels in underground rapid transit systems are discussed. Three types of cross-ventilation are considered, namely (a) numerous small holes in the dividing wall between the tracks in a cut-and-cover tunnel, (b) a curtailed dividing wall and (c) passages linking separately bored tunnels. It is shown that considerable reductions in the air velocities expected on station platforms will result if the end regions of the tunnels are well cross-ventilated. Additionally, useful reductions in the aerodynamic drag force on trains can be expected if cross-ventilation is provided along the whole length of the tunnel. However, this is recommended only for low speed systems because interaction between passing trains will cause large pressure fluctuations at high speeds.

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