Rainfall indexes for safe rail-transport operation control are required to show similar behaviors as these of the time-dependent slope stability of embankments and cutting during rainfall. Thus, we performed analytical investigations of this phenomenon, focusing on railway embankments. By setting models based on the data of actual railway sites, saturated / unsaturated seepage analyses were carried out under the conditions of patterned and measured rainfalls. Comparing with a series of safety factors obtained by slope stability analyses, we succeed in the characteristics of examined rainfall indexes.