High-speed railway standards, such as the Eurocode and UIC specifications, enable engineers to account for the constant moving loads of trains on bridges. The theoretical resonance speed of these bridges is calculated as the product of carriage length and bridge frequency. However, this approach may not always be accurate, especially for single-track steel bridges or when two trains travel in opposite directions. In these cases, the dynamic effects of the vehicle-bridge system are often overlooked, which can lead to overestimations of the bridge's resonance response or a drop in resonance speed. This paper proposes an analytical method for analyzing vehicle-bridge interactions to address these issues. It introduces calculation strategies that consider increased damping and additional beam modal mass, improving the accuracy of bridge response assessments under moving loads. This method effectively accounts for the frequency shifts that occur when a train crosses a bridge, which can lower the expected resonance speed. While Vehicle-Bridge Interaction effects can mitigate train-induced resonance, they can also cause frequency shifts, making it necessary to revise the nominal theoretical resonance speed. An analytical shift factor is proposed to facilitate this adjustment, which relates to the masses and positions of the bogies on the bridge. By using this shift factor, railway engineers can more accurately evaluate the resonance of high-speed railway bridges, even when actual speeds differ from theoretical predictions.