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