## 1 Introduction  
The rapid growth of tunnel mileage throughout the world greatly boosts the technical development of high-speed railway industry. Targeted operating speed of the railway high-speed train is constantly lifted up since the Shinkansen was put into operation in Japan in 1964. So far, high-speed train (HST) running at 350 km/h speed level in China railways has been achieved in many lines, e.g., the Beijing-Shanghai line and the Nanchang-Ganzhou line. With extensive efforts being paid in the last few decades, a further step regarding the acceleration of high-speed train velocity exceeding 400 km/h has been scheduled in many countries. Tunnel aerodynamic effect is one of the most important issues as it may lead to serious discomfort, noise and safety related problem. Extensive efforts have been paid in the last few decades, wherein the research interest can be roughly classified into two aspects, i.e., (1) the micro-pressure wave (MPW) emitted at the tunnel portal; and (2) the transient pressure load (TPL) acting at tunnel lining, internal facilities, and train body. [PERSON] et al. [1; 2] are among the first few investigating tunnel aerodynamic effect by utilizing both theoretical and experimental methods. Their research interest inclined to the reduction of MPW which was firstly observed at Sanyo Shinkansen laid between Osaka and Fukuoka. A fruitful study focusing on the tunnel aerodynamic effect was then carried out by [PERSON] et al. [3]. Investigation was made to explore the aerodynamic force and the shape design of train body, the transmission of pressure waves inside tunnel, and the impulsive wave discharged from tunnel exits, as well as its accompanied aero-acoustic problems. Research of their study related to MPW illustrated (1) the basic varying trend and the strength of MPW versus tunnel length and train speed; and (2) the control methodologies by promoting the design of train body, hoods and branches at tunnel exits or interior. However, findings of their research are bounded by the research background where values of train speed in data analysis are mostly around 200 km/h. Through theoretical analysis and scaled model tests, [PERSON] et al. [4; 5; 6; 7] systematically discussed the optimization of vented hoods by designing three type of tunnel hoods. Alleviation of aerodynamic effect was evaluated through plotting the peak pressure and pressure gradient versus train speed. Good agreement was achieved between the predicted the pressure waveform by theoretical models and measurements from scaled model experiments considering different tunnel hoods. Despite of studies based on theoretical concern, engineering oriented researches are conducted by many scholars, wherein parametric optimization of the tunnel and train structure is highlighted. By comparing field and scaled model measurements with numerical simulation data, [PERSON] et al. [8] analyzed both the transient pressure and micro-pressure regarding different blockage ratio, train nose, and train speed (220-380 km/h). Variation of the amplitude of MPW regarding the aforementioned parameters was primarily addressed. Through data analysis from simulation and model-scale experiment, [PERSON] et al. [9] studied the alleviation effect of MPW by using tunnel hoods that prevalently adopted in actual railway lines, such as the enlarged-transect windowless hood, gradient hood, linear horn hood, etc. Results imply that reduction rate within 20-50% is feasible for the most tunnel hoods by optimizing their design parameters. The most favorable condition appears at the linear horn hood (reduction rate = 71.6%) with the extended hood length \(L_{eh}\) being three times the equivalent diameter of the tunnel. Recently, [PERSON] et al. [10; 11] proposed a newly designed tunnel hood with multiple air-slits at both sides. Measurements presented by the 1/64.2 scaled tests show that for the train speed at 250 km/h, the maximum reduction rate of pressure gradient and MPW approaches 56.3% and 78.3%, respectively.