The guardrail is an indispensable part of ballasted track structures on bridges. In order to reveal its influence on the track–bridge interaction of continuous welded rail (CWR), the longitudinal resistance model of the guardrail fastener and its influential factors are established through tests. By taking a continuous girder bridge (CGB) for railways as an example, a stock rail-guardrail-sleeper-bridge-pier integrated simulation model is developed. The effects of the guardrails, installation torque of the guardrail fastener, and joint resistance of the guardrail under typical conditions are carefully examined. The research results indicate that the nominal longitudinal resistance of the guardrail fastener and the elastic longitudinal displacement of the rail prior to sliding approximately grow linearly with the growth of the installation torque. The presence of a guardrail can alleviate the track–bridge interaction in the range of the CGB, but exacerbate the interaction near the abutment with moveable bearings. This fact enables the abutment position to be considered as a new control point for the design of CWR on bridges. Considering the changing rules of the rail longitudinal force and rail gap, it is recommended that the installation torques of the guardrail fastener and guardrail joint are 40–60 N·m and 700–800 N·m, respectively. The recommended maximum longitudinal stiffness of piers for CGBs is evaluated. When the longitudinal stiffness of the pier for a CGB is lower than the recommended value, the influence of the guardrail can be neglected in the design of the CWR.

A novel prefabricated strengthening structure (NPSS) is proposed to improve the vertical stiffness and load-bearing capacity of existing short-span bridges for heavier axle-load trains passing through. The strengthening principle of the NPSS is revealed through theoretical derivation. A refined calculation model is prepared to investigate the effects of two important parameters on the structural behavior of the bridge, including the support stiffness and the installation location of the NPSS. The calculation model is also verified with four-point bending test of a bridge removed from a heavy-haul railway. With the calculation model and the response surface methodology (RSM), the functional relationships among the crucial mechanical indexes of the bridge and the two parameters of the NPSS are methodically established. Thus, the optimal values of the parameters are determined via a multi-objective optimization model and the analysis hierarchy process-fuzzy comprehensive evaluation method. Furthermore, the feasibility of the optimal parameters is appropriately verified based on simulations of the vehicle–track–bridge dynamics. The existence of the NPSS with optimal parameters could enhance the vertical stiffness of the bridge by 21.0% and bearing capacity by 19.5%. In addition, it could reduce the midspan dynamic deflection amplitude by 23.4% and vertical vibration acceleration amplitude of the bridge by 25.2%. The results of the study are expected to contribute to the capacity development and rehabilitation of existing heavy-haul railways with low cost and convenient construction without railway outage.