Based　on　the　Biot　theory　of　porous　media,　this　paper　proposes　a　coupled　vehicle-track-saturated　foundation　model　to　study　the　dynamic　responses　of　saturated　soft　soil　foundation　under　high　speed　train　via　two-and-half　dimensional　finite　element　method　(2.5D　FEM).　It　is　found　that　the　ratio　of　train　speed　c　and　soil　Darcy　permeability　kD(c/kD)　determines　the　extent　to　which　maximum　excess　pore　pressures　build　up　in　saturated　soft　soil　under　the　train　load　when　the　load　speed　is　lower　than　the　critical　speed.　For　a　saturated　soil　of　a　particular　stiffness,　if　c/kD　is　less　than　or　equal　to　3×104,　the　soil　can　be　viewed　as　highly　permeable　in　relation　to　the　load　velocity　and　almost　no　excess　pore　pressure　is　developed,　the　saturated　soil　can　be　modeled　using　single-phase　medium.　There　is　a　critical　value　of　c/kD　for　the　development　of　excess　pore　pressure,　when　c/kD　is　smaller　than　the　critical　value,　the　maximum　excess　pore　pressure　increases　with　increasing　c/kD.　Above　the　critical　value　of　c/kD,　the　maximum　excess　pore　pressure　remain　independent　of　c/kD.　The　amplitude　and　affect　scope　of　effective　stress　are　mainly　controlled　by　train　speed　and　soil　permeability　for　saturated　soft　soil　foundation.　The　response　of　displacement　is　mainly　controlled　by　the　train　speed.　Significant　Mach　effects　have　been　induced　in　the　saturated　soil　foundation　by　the　moving　train,　when　the　train　speed　reaches　or　exceeds　the　critical　velocity　of　the　track-foundation　system.　©　2021,　Science　Press.　All　right　reserved.