Apart from the construction clearance and the rolling stock clearance, aerodynamic effects should also be considered in the determination of railway tunnel crosssection. In this paper, the formulas of transient pressure in train, micropressure wave and air resistance are obtained based on assumption of onedimensional, compressible and unsteady flow. According to the indices of transient pressure in train, micropressure wave and air resistance, the existing design codes of intercity and highspeed railways are demonstrated and optimized. The results show that the crosssection of an extralong tunnel(more than 10 km) can be improved under the conditions of a certain length, seal level improvement of train and microbarometric wave buffer structure at tunnel portal.