Ultra High-Performance Fiber-Reinforced Cementitious Composite (UHPFRC) is increasingly popular for new structural designs thanks to its high resistance both in tension and compression. When UHPFRC is complemented with steel reinforcing bars or prestressing tendons, the structure remains waterproof and crack-free under service conditions, significantly improving the durability compared to conventional reinforced-concrete designs. The Aiguillon Bridge is one of the first railway bridges entirely made of UHPFRC. Built in 2021 in Switzerland, this bridge has a single span of 6.6 m and a width of 5.4 m. The bridge is designed for a narrow track and 2 walkways. Two prefabricated elements in UHPFRC have been assembled on-site by a longitudinal cast-in-place UHPFRC keying joint. This paper presents the full-scale laboratory experiment to validate the keying joint suitability for railway bridges. This experimental investigation involves a static test on a transverse beam composed of two precast elements and the keying joint. Results show that the structure presents a post-peak ductile behavior as well as an elastic behavior under service loads. Digital-Image-Correlation measurements provide details on the cracking patterns of the interface between the keying joint and the precast elements. As the maximum resistance obtained during the experiment is in agreement with the analytical-model predictions, the concept of the keying joint is validated.