AbstractA drive-by method is proposed to identify the modal frequencies of railway bridges from the dynamic responses of two-axle vehicles. The theoretical closed-form solution and frequency components of the vehicle response are derived based on the vehicle–bridge interaction (VBI) model composed of a two-axle vehicle and a simply supported bridge. The time-domain subtraction method (TSM) is improved and explained for two connected two-axle vehicles to reduce the effect of the track irregularity on the frequency identification of the bridge. The accuracy of the closed-form solution is verified by the finite-element (FE) analysis, indicating that both the vertical and pitching responses can be used for bridge frequency identification. Parametric studies are also conducted to investigate the factors that may affect the frequency identification. The results indicate that the TSM has a good performance and robustness against the track irregularity to ensure good frequency identification. The bridge frequencies with different stiffness can be well-identified regardless of the symmetry and damping of the vehicle, which shows a good prospect for indirect bridge monitoring.