The present study uses openFOAM package to simulate and investigate the aerodynamic performance and characteristics of high-speed trains (velocity beyond 250 km/h) under typical and critical conditions, which include flow passing a high-speed train, and two trains meeting towards each other at the same velocity in the open air. In terms of different operation conditions, separate openFOAM solvers are adopted. For flow passing a high-speed train at a constant velocity, a steady solver for incompressible, viscous and turbulent flow is employed on a fixed mesh and the results are compared with commercial software Star CCM+. For trains meeting towards each other, overset mesh method is used in which inverse distance interpolation is taken to couple background and inner overset mesh. The built-in mesh generation tool SnappyHexMesh is utilized to generate background and inner overset mesh. In all simulations, k-ω SST two equations RANS model is used to simulate the turbulent flow.