Bluff-nose trains with square edges tend to increase air resistance and pressure fluctuation when they enter a tunnel (single-track tunnels in particular) compared to those with rounded edges. This is because flow separation at these square edges increases the effective cross-sectional area of the train (Fig. 1).

The RTRI has proposed the size and shape of rounded edges at the front-end and the newly developed fins, in order to efficiently suppress the separation of airflow. Through model and field tests to evaluate the effect of the roundness, it was confirmed that it suppresses the large separation of flow, thereby reducing the maximum pressure gradient of compressive waves generated by the trains entering tunnels to about one third (Fig. 2). In a running test with a test train set fitted with fins (Fig. 3), it was also confirmed that the phenomenon of pressure overshooting does not occur by the train entry into the tunnel (Fig. 4) and that the running resistance is decreased by about 20% when the two-car train runs at 120 km/h. The design of the proposed rounded shape for the front ends of vehicles has been adopted for a few types of newly manufactured rolling stocks on meter-gauge railway lines.