Abstarct: Today, with increasing development of railway transportation industry in the world and our country, using high-speed trains are becoming more and more important. For this reason, planning and transport policies are trying to provide suitable condition to preparing the existing railway lines, to achieve this purpose. So the Current state of the railway must be carefully studied to identify their weaknesses points. The bridges are one of the important strategic structures in the railways. Bridges in terms of technology and in terms of operating costs are placed in a special place of importance. The structural health monitoring and retrofitting of railway bridges are the most important plans to provide railways for servicing the high speed trains. The purpose of this paper is suggesting a guideline for bridges to determine the maximum allowable velocity in high-speed trains. Because of the lack of appropriate codes and guidelines for this purpose for railway bridges and the growing need to develop and start using high-speed trains, need a capable instruction felt. This paper suggests a clear an applicable guideline for this goal that is explained in the chapter 4. As result, most railway bridges which were built about up to years ago, shows sufficient resistance against the high speed trains. In the case study the maximum vertical acceleration limited the maximum allowable velocity of high speed train. In this case however the maximum vertical acceleration exceeds the allowable value in the code and due to that the bridge loses its serviceability, but the structure still stay without damage. In these cases it is possible to increase allowable speed by some methods which controls middle span vertical acceleration.