Abstract: The analysis shows that the sizes, shapes and boundary conditions of embankment impact substantially on triggering the natural convective effect in ballast embankments. Pressure boundary conditions of ballast embankment are dependent on engineering conditions. The numerical results on ballast embankments with four pressure boundary conditions show that the natural convection cooling effect in ballast embankment with air-permeable boundaries is the strongest. A natural convection index for representing the cooling effectiveness of winter-time natural convection in ballast embankments is introduced. The approach to evaluate ballast layer heights in railway embankments in cold regions is developed. Evaluating ballast layer heights of embankment with the grain sizes of 2cm~4cm, 4cm~6cm, 6cm~8cm and 8cm~10cm is performed. Validity of ballast layer heights with the grain size of 6cm~8cm was demonstrated experimentally. The test results show that the ballast layer heights in railway embankments evaluated by the natural convection index are available for design of ballast embankment.