Among different details in old riveted railway bridges throughout the world, connections such as stringer-to-floor-beam double-angle connections are one of the most fatigue-prone details. These connections are subjected to fatigue loadings due to the secondary out-of-plane deformation. However, there are only few traditional strengthening techniques that are proved to be not so effective in permanently addressing the fatigue issue. In this study, a novel un-bonded prestressed retrofitting solution is proposed for strengthening riveted/bolted steel angle connections. The strengthening system consists of prestressed carbon fiber reinforced polymer (CFRP) rods, which are clamped at the ends using mechanical wedge-barrel anchors. The prestressing force of CFRP rods are transmitted to the strengthened member using the clamps functioning purely by friction. Therefore, an adhesive bonding is not required, and, no damage is imposed to the parent structure. The installation procedure consists of two main stages: presetting the wedges into the barrels, and, prestressing the CFRP rods. For the presetting, a novel method is introduced, making it possible to apply high presetting forces on-site. The application of prestressing force is done by pumping the hydraulic jacks. The static behavior of the strengthening system is experimentally investigated through pull-off tests. In addition, the fatigue behavior of the strengthening system is studied by the laboratory tests on stringer-to-floor-beam double-angle connections. Furthermore, stress distribution in the clamping system and connected parts was analyzed using a Finite Element (FE) model leading to final design of the system.