Abstract A new method of optimization of the N MV is proposed. The comfort index N MV was measured and analyzed in research carried out for the Greek Railway network. Factorial analysis was used for the calculation of the effect of each factor on the comfort index N MV. ISO 2631-1 standards were also involved in this study for the design of the equivalent vibration dose lines as a function of the N MV and the exposure time. Using the factorial model, the N MV index for different values of a train’s speed and rail condition was predicted. Finally, an optimum speed value, as regards acceptable passenger comfort, was obtained by using the equal vibration dose lines.