The paper deals with diagnostics and calculating the load-carrying capacity of the prestressed concrete railway bridge near the town of Stará Ľubovňa, the northeast part of Slovakia. The bridge object was probably built between 1965 and 1966 because the original project documentation was drawn up in 1964. The bridge is located on the Orlov-Podolínec railway line at 21.3 km. The bridge is made from precast prestressed post-tensioned box girders of three single spans. In the ground plan view, the bridge object is curved. In the transverse direction, the box-girder consists of two longitudinally connected parts connected in the transverse direction using transverse prestressing. The overall cross-section is a box-girder with 3 openings and 4 vertical walls. In 2023, in cooperation with practice, the Department of Structures and Bridges, the Faculty of Civil Engineering, University of Žilina, was asked to carry out the technical survey and diagnosis of the mentioned bridge. Visual inspection, diagnosis, and verifying real dimensions and material characteristics were requested. Non-destructive and semi-destructive testing methods were used to determine the material properties of concrete, reinforcement, and prestressing steel. After that, the load-carrying capacity calculation was done. For this purpose, a numerical 3D FEM model was created. The standard approach was used to determine the load-carrying capacity according to provisions, which consider the modified (lower) reliability levels and their adequate partial safety factors according to Eurocodes. The results of the diagnosis and the calculation are presented in the paper.