This paper presented a new retrofit system for strengthening the stringer-to-floor-beam double-angle connections in a 92-year-old riveted railway bridge in Switzerland, using prestressed carbon fiber-reinforced polymer (CFRP) rods. The strengthening system transmitted the forces purely through friction, with minimum interference with bridge traffic. The system consisted of two components: a newly developed mechanical wedge-barrel anchor to hold the prestressed CFRP rod and a clamping system attaching the stringer flange. The strengthening system reduced the out-of-plane deformation of the connections, resulting in a reduction in the distortion-induced stresses in the connections. The short-term bridge measurements showed that the application of a total prestressing force of 100 kN reduced the dominant mean stress in the connection hotspot by 47% (from 22.9 to 10.9 MPa) under the passage of passenger trains, whereas the stress range remained unchanged. Using the modified Wohler curve method (MWCM) as a critical plane-based multiaxial fatigue model, the strengthening system was observed to reduce the multiaxial mean stress parameter by 30% under the passenger train loads. The long-term monitoring of the strengthening system, using a wireless sensor network (WSN) system, indicated no prestress loss in the CFRP rods during the seven-month period since its installation.