Vacuum Tube Transportation (VTT) system utilizes the low-pressure environment to achieve aerodynamic drag reduction and reduce the energy consumption of high-speed trains. Within this system, even for trains operating at subsonic speed, the flow acceleration around the streamlined train may leads to local supersonic flow. The appearance of the supersonic flow will dramatically alter the flow field structure and aerodynamic load of the high-speed train. Therefore, this study investigates two types of representative flow fields under different blockage ratio through the improved delayed detached eddy simulation (IDDES) method. The initialization, propagation, reflection and intersection process of shock waves in transonic case are captured and analyzed. It is shown that, the boundary layer thickness and width of vortex group in wake has experienced a slight increase with the increase of vacuum level. Compared with the subsonic case, the occurrence of choked flow and shock wave in transonic flow field lead to a decrease of wake region's vortex group width. As a result, the total drag coefficient of the train is increased by approximately one order of magnitude, and the dominant St number of tail car's drag coefficient has shifted from 0.16 to 0.11.

真空管运输（VTT）系统利用低压环境实现气动减阻，降低高速列车能耗。在该系统内，即使对于以亚音速运行的列车，流线型列车周围的流动加速度也可能导致局部超音速流动。超音速流的出现将极大地改变高速列车的流场结构和气动载荷。因此，本研究通过改进的延迟分离涡模拟（IDDES）方法研究了不同堵塞比下的两种代表性流场。捕获并分析了跨音速情况下激波的初始化、传播、反射和相交过程。结果表明，尾流涡群边界层厚度和宽度随着真空度的增加而略有增加。与亚音速情况相比，跨音速流场中阻塞流和激波的出现导致尾流区涡群宽度减小。结果，列车的总阻力系数增加了大约一个数量级，尾车阻力系数的主导St数从0.16变为0.11。