The operational speed of the trains is intended to be significantly increased forthcoming; which consequently questions the safety level of the current design concepts concerning different performance limits. Thus, the reliability of the simply supported single-span bridges is assessed in the current article adopting the first-order reliability method (FORM) approach. In this regard, the dynamic response of the aimed bridges is investigated under the passage of a series of moving loads using available closed-form solutions in the literature. Hereof, axle load, car body/train configuration, flexural rigidity, damping, mass and model uncertainties are considered as random variables; while train-track-bridge interactions are neglected and the procedure is repeated for a wide range of span lengths and train velocities. Then, the safety index corresponding to each case is evaluated by considering running safety as the limit state function; where, the bridge deck vertical acceleration is taken as the capacity of the system. The outcomes are presented as average probability of exceeding the limit state versus train speed and categorized based on span lengths.