As a train runs at higher speed, aerodynamic drag becomes larger, which should be reduced from the viewpoint of saving energy. Previous researches show that it is the most effective to smooth the under-floor construction for this purpose. However, most researches in the past have not approved the precision of estimation on the reduction of aerodynamic drag of trains. In this paper, we estimate the reduced quantity of aerodynamic drag by using the Maibara Wind Tunnel designed for railways, compare it with that obtained from on-track tests and verify the precision of the estimation. Then, we clarify the effect of bogies and each part of vehicle body on the aerodynamic drag by improving the under-floor construction.