Abstract: Whether the piston speed fast or slow, an ordinary piston can generate moving shock wave while it moves in a tube. The hollow piston is different from the ordinary one. When the movement Mach number of a hollow piston is lower than the choked Mach number, the moving shock wave will not appear; only when the movement Mach number is higher than the choked Mach number, the moving shock wave can be generated. The choked Mach number can be determined by diameter, length, averaged friction coefficient of inner hole and outer diameter of a hollow piston. Train in tunnel is similar to hollow piston in tube. According to the shape of cavity between the tunnel wall and the railway carriage, an equivalent hydraulic diameter can be obtained. Then the conditions of generating moving shock wave can be computed by a similar method for calculating choked Mach number of hollow piston. The novel method for measuring averaged friction coefficient of a train was proposed.