This research investigates the influence of fouled ballast layers on vibrations in railway bridges. The research introduces and validates a Train-Track-Bridge Interaction (TTBI) model designed to investigate the dynamic response of a bridge under different degrees of ballast contamination. The primary focus is on analysing midpoint deflection, velocity, and acceleration during the traversal of a train. The study reveals substantial improvements in track stability with a 37.5% reduction in bridge deflection as ballast stiffness increases from 40 to 70 MPa. Conversely, contaminated ballast leads to a 35.7% increase in midpoint velocity and a doubled vibration range when ballast stiffness rises from 40 to 80 MPa, highlighting the importance of considering ballast conditions in railway infrastructure design. Additionally, there is a significant 28.6% variation in maximum acceleration with increasing fouling rates, particularly impacting the bridge during the crossing of the 2nd wagon. The findings underscore the critical necessity for robust maintenance strategies to address fouling issues and ensure the long-term health and performance of railway bridges.