In order to perform proper and effective frequency analyses of railway noise, easy monitoring of noise level variation and easy selection of the analyzing time span simultaneously are indispensable when calculation to obtain power spectra etc. is executed. This report introduces a newly devised system of analyzing time series data and several analyzed samples. The system is made adaptable to frequency analyses of actual railway noise and enables a reasonable change from an analogue method of noise level evaluation under the SLOW or FAST time constant to a digital one of computing the power average level.