Repeated　train　passages　bring　detrimental　effects　on　train　operations,　especially　at　high　speeds.　In　this　study,　a　computational　model　consisting　of　moving　train　vehicles,　track　structure,　and　track　foundation　is　used　to　investigate　the　stress　distribution　in　the　track　substructure　and　underlying　soil,　particularly　when　the　train　speed　approaches　the　critical　speed　via　2.5D　finite　element　method.　The　numerical　model　has　been　validated　by　in-situ　test　results　from　a　ballasted　high-speed　railway.　The　computational　results　reveal　that　the　substructure　is　shown　to　be　effective　in　reducing　the　stresses　transmitted　to　the　ground;　however,　a　simple　Boussinesq　approximation　is　proved　to　be　inaccurate　because　it　cannot　properly　take　account　of　the　effect　of　multi-layered　substructures　and　train　speeds.　It　is　acceptable　to　assume　a　simplified　smooth　track　in　the　analysis　model　for　determining　the　maximum　stresses　and　displacements　for　a　low-speed　railway　(＜=　100　km/h)　but,　for　a　high-speed　one,　the　dynamic　amplification　effect　of　track　irregularities　must　also　be　considered　in　subgrade　design.　Analysis　of　the　stress　paths　revealed　that　the　load　speed　and　track　irregularity　increase　the　likelihood　of　failure　for　the　subgrade;　track　irregularity　can　induce　many　times　of　principal　stress　rotations　even　under　a　simple　single　moving　load.