A numerical simulation technique is developed to analyse the train-track-bridge system dynamic responses when a train traverses a bridge. The basic idea of the simulation is to consider the train, the track and the bridge as a whole interactive system. The modelling aspect of the train-tack-bridge system is presented. A large-scale software, TTBSIM, is designed to simulate the dynamic responses of the complete train-track-bridge system so as to assess the running safety and the ride comfort of trains traversing bridges in the design stage. Two high-speed running experiments were carried out on the Qinhuangdao-Shenyang line, which were used to validate the TTBSIM technique. Results show good correlation between simulation and measurement. TTBSIM software has provided a dynamic assessment tool for the design of bridges on new high-speed railways.