As a train runs at higher speeds, an aerodynamic drag increases. On long train-sets such as those of Shinkansen, the aerodynamic drag is mainly generated by intermediate vehicles. In previous researches, we proved that smoothing the under-floor construction reduces the aerodynamic drag. To investigate the mechanism of the effect, we performed wind tunnel tests with train models consisted of three vehicles, each representing a head, intermediate and tail vehicle, and measured the aerodynamic drag and pressure distributions on the intermediate vehicle. Test results show that the reduced aerodynamic drag is mainly the effect of decreases in the pressure drags around bogies.