The vibration of simple and three-span continuous beams traveled by trains moving at high speeds is studied in this paper. Central to this study is the adoption of a dimensionless speed parameter S, defined as the ratio of the exciting frequency of the moving vehicles to the fundamental frequency of the beam. The numerical studies indicate that the moving load model is generally accurate for simulating the bridge response. However, the use of the sprung mass model is necessary whenever the riding comfort of rail cars is of concern. If the characteristic length, rather than the span length, is used for the continuous beam, then both the simple and continuous beams will reach their peak responses at the same critical speed S, when traveled by wheel loads of constant intervals. The rail irregularity, ballast stiffness, suspension stiffness and suspension damping can drastically affect the riding comfort of rail cars traveling over simple beams. Their effects are comparatively small for continuous beams. In conclusion, the design of a high speed rail bridge is governed primarily by the conditions of serviceability, rather than by strength.