This paper proposes a prediction method to estimate vibrations in building structures induced by to-be-constructed railways. The method derives modal equations of motion for building modeling by using the natural frequencies in limited modes, the corresponding damping ratios and mode shapes at selected locations. The participation vector for input ground motions is approximated by the corresponding mode shapes. Those modal properties can be obtained from ambient vibration testing, and hence the modeling does not require design documents to construct the mass, damping and stiffness matrices. The railway-induced motions at the column bases can be estimated priorly, and the dynamic responses at the selected locations are subsequently predicted by mode superposition. The fundamental characteristics of the proposed prediction method are studied by numerical simulations. The simulation result shows high participation-vector approximation accuracy when using few lower mode shapes at few locations; Higher modes exhibit more complicated mode shape amplitude distribution, and thus mode shapes at more locations are required for the sufficient approximation accuracy; Compared with seismic response calculation in the horizontal directions where few lower modes are sufficient, higher modes are required for satisfactory prediction accuracy. The method provides an alternative approach for predicting railway-induced vibrations and is helpful when design documents of the objective building are unavailable. The findings from the simulation provide guidance to the measurement-point design and the objective modes to identify in the implementation of the proposed method.