The characteristics of dynamic water pressure in transient saturated zone of red sandstone residual soil subgrade is the key to deeply understand the mechanism of repeated mud pumping under the continuous rainfall condition. Based on the undrained dynamic triaxial test

the dynamic water pressure characteristics of the subgrade transient saturated area are studied by simulating the train loading frequency and the drainage boundary conditions under the condition of continuous rainfall. The influence of fine content on dynamic water pressure in transient saturated zone of red sandstone residual soil subgrade is emphatically analyzed

it is found that the critical value of fine content is about 25%. When the fine content is less than 25%

the dynamic water pressure increases with the increase of fine content. When the fine content is greater than 25%

the dynamic water pressure decreases with the increase of fine content. The concept of equivalent intergranular contact void ratio is introduced to clarify the physical significance of the critical value of fine content. An empirical model of dynamic water pressure in transient saturated zone of red sandstone residual soil subgrade is established based on the experiment and the sensitivity of model parameters is analyzed. The model is helpful for engineers and technicians to predict the dynamic water pressure in the transient saturated zone of railway subgrade under continuous rainfall.