When a train passes a conventiaonl tunnel at high speed, external pressure variation problem arises. It is known that this issue can be reduced by control the tunnel length. We studied the variances of external pressure variation within the tunnel, by altering length of the dummy tunnel duct. We also studies the variances of micro-pressure waves at the exit of tunnel, by altering surface area of dummy tunnel duct. For analyzing this train-tunnel relation problem, axisymmetric steady compressible flow solver was used.