In China, a new railway middle-span emergency steel truss girder, which can adapt to the technical requirements about quickly repairing the damaged railway bridges, is developed under the new situation. The old emergency steel truss girder for the railway bridges can not adapt to the current technical requirements. So, the new railway middle-span emergency steel truss girder (hereinafter referred to as the new girder) is developed after hard work. The great changes on the structure, the connections, the structural details and the multi-purpose have taken place on it compared to the existing old similar girder. In this paper, the structure and technical characteristics of the new girder are introduced. In order to improve the new girder’s design, the dynamic responses of the new girder are calculated using the train-bridge coupling vibration theory. In the paper, the 32m-span girder are the main object of study. The vibration-related laws are revealed versus the different train speeds, and the proposals about the largest traffic safety speed are given.